Or what, they'll get fired? Could a company do so without risk of a lawsuit?

Yes, it was probably part of their employment agreement, that all public appearances are at the approval of Google. That's pretty typical, even for an acquired startup.

And if they get fired with cause, they most likely loose a lot of their acquisition payout, so it is in their best interest to follow the rules.

And these guys are YouTube founders - pretty sure exceptions can be made for them. Also, they deserve to be in the photograph considering the relevance.

> And these guys are YouTube founders

And once they were acquired, no different than any other Google employee. That is the cost of "selling out" as they say.