if maintaining a positive relationship with another company is more valuable than the hire, (and when is this not the case?), obviously you will clear it with the other CEO first. I haven't read the book but you haven't quoted enough context to label this disgusting.
There's a difference between 'do not poach' and 'do not hire'. A 'do not hire' list is in essence a Blacklist, which if employed between companies with overlapping skill sets (IE trying to block an engineer moving between Apple and Microsoft) then this is certainly disgusting.
Poaching is the active targeting of employees of other companies, which when done in the umbrella of an investors holdings is bad form (don't bite the hand that feeds you). When done to another company is generally a bad idea if you expect to have to have amicable dealings with them (license patents, buy parts, etc). However, if they're a direct rival and there's no foreseeable reason to be on good terms with them, there is no reason to not poach their employees other than the reason you know you're not acquiring a loyal employee, but one loyal to the money.
A 'do not hire' list is an attempt to blacklist anyone who essentially leaves whatever the umbrella is of its reach, which is an attempt to place a monetary cost on anyone trying to leave the corporate fold. It would be like a bartender breaking the legs of anyone trying to leave. Anyone else who was planning on leaving isn't going to, but they're certainly not going to be buying drinks and that's really going to drive down profits when the bar is at capacity and no one is drinking. There's a reason companies are employing "quitting" incentives, and that's for the very reason that happy employees are productive employees and the notion of one bad apple spoils the bushel.
So lets pretend I am a tech lead at a AcmeCo, I quit and start a startup, and an old teammate wants me to hire him. But he is an important employee at AcmeCo and my old CTO will be unable to replace him & his knowledge. I care about my relationship with AcmeCo & the CTO in particular, he is a potential client, we share ownership of some open source offerings, he still mentors me.
How could I possibly take any action that damages this relationship?
