He didn't just mass downvote. He purposefully attempted to remove other contributing members from the community. He also did not confess to it indicating both dishonesty and that he was aware that his actions were unacceptable. He also multi-accounted and still does and posts absolutely disgusting and logic-free racial comments and trolling (referring to black scientists to "dancing bears." You're welcome to demonstrate what's rational or constructive about that).

You don't just undo those actions, you punish the person who takes part in them in order to deter the action occurring in the future. So that there can be civil discourse going forward. This is rational and a standard part of human social requirements.