Everybody else on the team knows what they did, that they flat out betrayed them. Damian shouldn't have to be the one that has to bring the hammer down on them. The whole team should bring the hammer down on these two.
There's not wanting to leave things in Damian's hands, and then there's betraying your team and joining forces with the dude who's come to the future to murder Superman's 10 year old son. Its not like theres no previous context that this Tim is an unstable bad dude either. I blame them and they should be held accountable for this. Not just by Damian but by everyone.