It's the ridiculousness of what is considered ok and what is not by the censors. Ass = whole bottom region (generally ok in art and language without censoring), asshole=orifice (oh noes /s), like this fuck, fucker, etc = profane but not obscene slang term for the act or the person committing the act of sex, mother fucker = someone who engages in incest.
Same reason why they had to edit down Boys Don't Cry to remove the "evidence" of the girlfriend's pleasure, because all the cum jokes in comedy moves are perfectly fine (Something About Mary, Van Wilder, etc. etc. etc.) but show someone with a wet mouth after going down on a girl and it's suddenly a step too far.
I was shocked when they started dropping curses in commercials in the US. The first one I noticed was when I was in HS and I called up one of my best friends (super religious family, they moved away like 6 years before but homies 4 life yo) and I was like... HAVE YOU SEEN THIS!?
Same reason why they had to edit down Boys Don't Cry to remove the "evidence" of the girlfriend's pleasure, because all the cum jokes in comedy moves are perfectly fine (Something About Mary, Van Wilder, etc. etc. etc.) but show someone with a wet mouth after going down on a girl and it's suddenly a step too far.