Why are there "curse" words if profanity isn't illegal?

our speech cant be filtered and the only thing that deems a word "bad" is society, so why do we clify a four letter word meaning , or poop as unnacceptable, the only reasons these words have power is because we give them power by clifying it as a "curse" or "swear" its not religiously unethical, it doesn't take the lord's name in vain (vein?)