Why is ass considered obscene?
When ass is used to refer to the animal, it's generally not considered obscene. Some would consider it obscene when referring to the human posterior, or more likely, when used as an epithet for a boorish or stupid person. Asshole in my view is considered even more obscene.
The "why?" question is pretty subjective. Why is any word considered obscene? It's just the way the word developed and societal convention.
I believe in the spectrum of obscenities, even as an epithet it's not that obscene. In my opinion, a filter which replaces the word with butt is too stringent.
What is the difference between ass and butt?
When referring to anatomy, there's little difference. Each word has other senses, however. An ass can refer to the sure-footed animal smaller than a horse with big ears. It can also mean a pompous fool or be used as slang for sexual intercourse (get some ass).
Butt has many other senses. It can be the part of the trunk of a tree nearest the roots, the victim of ridicule (butt of your jokes), thick end of a handle or stock of a rifle/pistol, the end of a cigarette, or a type of joint (carpentry, not the other type of cigarette.)
Butt can also be used as a verb, to place end to end, to lie adjacent, or to strike or shove against/in between, (to butt in line.)