in fact what are the rules regarding obscenities?
The rules are that there are no rules. Different people find different things offensive. The word "shit" in and of itself is no more offensive than any other word except that it has a history of being used in derogatory and profane contexts.
Since you find a particular word offensive, the onus is on you to avoid that word or ignore posts by people that use that word or (as your shameless plug suggests) filter that word from text that uses it while not curtailing my right to read that word in its proper context.