Because criticizing those in authority is very different from the opposite. The tired old dead horse of "its technically also racism to criticize the plantation master!" is an unworthy argument.
The "punching up is okay!" excuse is only useful to people who are in it for the punching.
How about if nobody gets punched. Don't roll out demeaning stereotypes against any groups; instead, treat people like human beings. Who knows, it might work out!
