The rationale (which, again, I'm not arguing for or against) is that mocap performances are not strictly speaking totally the actors, because mocap has to be cleaned and can be (and very often is) edited and tweaked after the fact by animators. Not to mention there are often required liberties taken because a model cannot line up one to one with an actor anatomically.

In a sense, mocap performances are done by a team of animators where one animator puppeted a model in real time.