Well, the commissioner (who I seem to recall is a lawyer) is trying to clean up the league's image. He's suspended players for DUI, drug offenses, domestic violence, etc. He's hammering on the point that playing in the league is a privilege, not a right. If he were wrong about that, I suspect we'd see lawsuits left and right by now.
What Vick did was despicable. Is it fair to suspend him 4 more games? I don't think so. I do think the conditional admittance is fine...keep him on a short leash (pun intended). But, if it wasn't legal, the practice would have been challenged long ago.
The tragedy is that Vick harmed or caused to be harmed a lot of innocent animals. He also pissed away a fortune, his reputation, etc. Whatever else happens, I have no sympathy for him. He's a thug, and he did this all for sport. What an idiot.