It's often felt and stated that Rocco betrayed Michael in II, that he was the one who killed the would-be assassins. Why is this so assumed?

People seem to take Michael sending him on his suicide mission, and Rocco accepting it, as Michael offering Rocco a chance to atone, and Rocco accepting it is him accepting the offer of an "honorable death" ala Pentangelli - but could it not just be Michael didn't really care if Rocco came back alive or not; he was just a soldier, small potatoes, a pawn, a useful tool. He wasn't Al Neri. Rocco was a good soldier, and what he was going to do - say no? He saw Michael was ready to take out anyone he saw as potential enemy, lashing out publicly at his own "brother" Tom. So, you'll say no to that man?

It's stated in the film that Fredo had a crew. So, could it not have been one of "Fredo's people" who killed the assassins?

Perhaps Rocco failed in bringing the would-be assassins back alive, but Michael already figured they were dead before he ordered them to be be found alive. "If I'm right, they're dead already." So I don't think he sent Rocco on a suicide mission to atone for a failure of something he foresaw as being inevitable, anyway.