Can't upvote this enough. Facebook in NYC does this also, and it's just plain creepy.
Mostly because it would take them no effort at all to advance this policy in advance. But they don't -- they just ambush you with it when you get there. And they know damn well that no one's going sit down and read (let alone think about the implications of) what they're signing. That's the creepy part.
This is quite silly, of course. A solid developer has no problems switching idioms (not just syntax) when hopping languages.
There's really not much sense in an argument that says you "lose" something by learning newer things -- provided, that is, you're really making an effort to wrap your head around the newer things that you're studying, and not just cramming selected phrases and reflexes into your head in order to finish some current task at hand.
If anything he's become a better C# programmer, not a worse one, precisely because of his "bad" experience, because he now sees that overuse of certain habits can lead to bizarre or otherwise undesirable results (just like with any language).
So really he should be thinking positively about the whole experience, rather than negatively.
And I bet that almost none of the companies who've just posted in the Who's Hiring? thread would have hired a young Mr. Zuckerberg on the basis of his coding chops (let alone his personality) either.
Mostly because it would take them no effort at all to advance this policy in advance. But they don't -- they just ambush you with it when you get there. And they know damn well that no one's going sit down and read (let alone think about the implications of) what they're signing. That's the creepy part.