It’s as simple as noting that new users are new and still learning from everyone else around them. When everybody is new, it’s hard to get a feel for the current culture and style.
I imagine that it would probably work on a permanent basis to gate new accounts to a certain fixed value per day or proportion of new to old accounts, e.g. .1% per day or whatever.
Personally, I came here from Reddit 1,791 days ago, so I will be the first to say that I hope we don’t have an “anti-Redditor refugee” policy, whether express or implied. Far better to demonstrate through action and voting what we value as a culture and welcome as many people to the site as we can handle without diluting our ability to teach by example.
More could be done to explain the culture in addition to what is now on newsguidelines.html and the faq.
This is almost surely an example of euphemism in the service of politeness. My favorite example of Paul Graham wording in the service of saying something firm nicely is "When disagreeing, please reply to the argument instead of calling names. E.g. 'That is an idiotic thing to say; 1 + 1 is 2, not 3' can be shortened to '1 + 1 is 2, not 3.'" I love the use of "can be shortened to" there, as if pg's only concern is saving people extra keystrokes. I think the polite phrase "stealing users" here similarly leaves another idea unexpressed.
The relevant thing here is not whether pg's stated motive is consistent with his actions, let alone whether reddit will appreciate his actions, but whether the mind-states corresponding to that motive are the most probable of those that would result in his saying what he did.
A good reason to question his motive is the fact that HN and reddit are not necessarily substitutes for each other; another is the fact that he's well aware (as others have pointed out) of differences between the two communities that he wants to preserve.
...but the two aren't mutually exclusive. After all, if Reddit users permanently migrate here, it actually does mean that Reddit's owners and admins are paying for their stand.
one is for fun, one is for pedantry
All in good fun, though, I suppose :)
Of course, the main difference is that allowing users to register would be a passive act, whereas Mashable is actively "profiting" on the SOPA black-out.
I don't know whether it was for diplomacy or not (it could be but only pg can answer that). But if it was done for a reason other than the stated "stealing" what's wrong with just saying "we have heavy volume today because of other sites that are down including reddit so I decided to turn off new account creation".
That's honest and avoids any chance of double speak.
Edit (based on comment below which I can't reply to):
Saying that my statement is honest is not the same as saying what was done is not honest.