I think keeping the title on Ambrose is just convenience at this point. It's not like the odds are extremely high or anything that they'd be doing much with the belt if it was on someone else anyway considering they can't even be arsed to book Ambrose (one of the more important members on the roster) in an actual feud over the gold, so they may as well just leave it on him for the time being. At the very least, It can be used to add further discord to the group later on when he does lose the title (he can blame Rollins and Reigns for the loss if they refuse to accompany him to ringside as his muscle for an important title defense.)