They did it because in their filthy littl sex-infested minds, they actually believe that people reading that Jesus rode into jerusalem on an ass are giggling behind their hands.
As we all know jesus rode into jerusalem on a DONKEY not an anal sphincter. If they make this change in other languages where "ASS in not a synonym for "arsehole," they will make ASSES of themselves, if not Donkeys and ARSEHOLES.
HAHAHAHAHAHAHAHAHAAHAHAHAHA
HB