Surely there were other developers who had "side projects" they really loved who happily went with Zynga. I really have to question why this guy was the _only_ one who didn't jump ship. Sure you can write a post claiming to be the white knight and not another corporate zombie like the rest, but that's very rarely the case.
I see this as a power play. Instead of being a "Zynga acquisition", I think he saw it as much more beneficial to be the David vs. Goliath. Good for him, I guess.