Parnas motive here is that Trump burned him, and he was really sold on the idea that they were all in it together, and untouchable. Same as Cohen basically.
Plus, how it works in Ukraine/Russia is Putin or whomever, would just make sure they were released, and sent home, but Parnas getting up close with the Southern District prosecutors, he realizes no one is going to help him, and this is his best move. He could probably take the case to the chin and do 18 months, and forfeit a million, but these guys are businessmen, and don't take loyalty oaths, its pure pragmatism.