Having said that, I want to play devil's advocate for a second. It's just my natural reaction when the press embeds big unquestioned assumptions in reporting for the sake of sensationalism -- namely that this is obviously "illegal" and nefarious behavior. Because, after all, it kept down wages for people like me who might otherwise have been more actively recruited.
If your reaction is OF COURSE IT'S ILLEGAL, just hold on a second. Try to think about the counterargument rationally, even if it's against your personal interests. (That's what I'm doing.)
There do exist many precedents for agreements and restrictions on seemingly "competitive" behavior that is deemed "unfair".
Let's consider a related example from another domain: countries "dumping" cheap goods into foreign markets. For example, China is accused of "dumping" subsidized steel into the U.S. market, which hurts the industry here. The U.S. steel industry works with the government to put in place tariffs that block the practice.
So is the U.S. steel industry being anti-competitive here? Have they engaged in an anti-competitive pact with the U.S. government conspiring to keep up the price of steel here?
Well, on the surface, yes they have. But the distinction is that "dumping" is meant to describe a particular form of competition that is short term and unsustainable in nature and designed primarily to destroy competing business. It is meant to isolate a specific practice that actually hurts competition in the long run.
So, what about the practice of poaching employees from competitors? Well think about it for a minute.
What is the value of a Safari engineer to Google's nascent Chrome team? The truth is, it's a lot more than just that engineer's skillset. There's also the value of undercutting Apple's Safari development. That's HUGELY valuable to a competitor.
So, I just wanted to put it out there that at first blush at least, that the practice of "poaching" may actually have an anticompetitive angle. Just like with "dumping", you have to look beyond the immediate act (restricting/boosting prices) and examine the particular practice it's attempting to curtail.
One way to look at anti-competitive rules is that the point isn't to protect companies at all. In the steel case, the point is that dumping will destroy the American industry and then--this is the crucial part--China can hike up its prices way above current levels because there's no competition. If China couldn't do that for whatever reason, then that would be great for American consumers, who would benefit from cheaper steel, and--in principle at least--the competition rules would do nothing about it. In other words, in theory, the law doesn't care about the American steel industry; it only cares about the long-term well-being of steel consumers, which requires sane pricing. That industries have co-opted that in all sorts of ways to ironically entrench their positions in anti-competitive ways should not distract you from the core purpose of the rules.
So applying that here, since the rules in question are meant to protect the workers, the problem would be if Google were hiring all these people to destroy Apple and then be able to slash salaries, since no one would have anywhere else to go. Since that strikes me as implausible, and at the very least not established, I can't see how we can think of Google's poaching as anti-competitive.
 I'm not sure I buy the US steel argument either, but that's a separate discussion.
But to complete my devil's advocate argument, "poached" offers could in fact be unsustainable in the long run. It may be worth it to Chrome to add a $100K signing bonus to a Safari engineer. But they're not going to be paying them an extra $100K above market year over year.
If there actually were a poaching war kicked off, you can imagine that the value of inflicting targeted damage on competing teams, and protecting teams from such damage, could balloon to a very large amount. Theoretically all the way up to expected profit, if there were no other mechanisms to prevent it.
This could have a chilling effect especially on startups entering competition with deep-pocketed big companies. They could just poach the best talent with offers the startups couldn't match. That's exactly like a big chain engaging in loss-leader tactics to destroy local competition.
You could maybe also draw a parallel with what's going on with patents right now. It's a huge tax on development, especially to startups.
Now that I think about it, this could very well backfire on all of us. Anti-poaching agreements among companies aren't the only way to prevent this sort of thing. If they get sued over that (which is targeted at the practice of poaching), what companies may start doing is figuring out more restrictive covenants in employment contracts. Exclusivity clauses, etc. I'm a little scared to think what they may come up with and standardize if their attention is focused on it.
I agree that would be seriously problematic and run afoul of at least the intent of the rules, but I haven't actually seen any evidence that it actually happens that way. And in the meantime, anti-poaching agreements clearly do undercut salaries; that's why they were--at considerable risk to the companies--implemented in the first place.
On your last paragraph, I have two thoughts: a) at least that would be above-the-board and could be evaluated by potential employees, and b) while employment law varies considerably state by state, it's generally not the case that the employer can enforce arbitrarily restrictive clauses. I know in Massachusetts, enforcing a non-compete requires meeting some reasonability requirements  and AIFAK California is even more pro-employee on those things. Just because you sign it does not make it so.
The problem is you seem to think it's only additive and therefore great for employees. Hey, more bonuses, more equity.
But if everyone adopts it as a standard operating procedure, it could lead to a lowering of base salaries. So then we get paid like salespeople: little salary, mostly performance based. Do you want that?
Bottom line, it could be opening a can of worms. My personal philosophy is that it's rarely good to be righteous and inflexible. It often backfires or escalates a situation.
If the execs didn't think what they were doing is bad, why did another major CEO refuse citing that it's probably illegal? why did they take efforts to hide it from the public and employees?
Tariffs aren't done in secret, nor is there a lack of public input on the practice.
There should be no issue in people hiring from each other and thus no need to make agreements behind everyones backs.
These internal policies did not restrict hiring for the most part, they just restricted cold-calling and active poaching.
Mostly, the last thing any of us wants is a select group of dominant companies making a collective decision for the industry.