I agree with you that keep things constructive is a great ethos, given the gist of the community. But shouldn't the don't be an asshole rule more or less cover any instances where obscene language would get in the way of constructiveness?
If somebody replies "fuck you" to one of my comments, I'm fine reporting it if I want to; or it'll be removed by a mod for breaking the asshole rule. If somebody simply drops an f-bomb in the comments in a non-insulting way, I'm not sure it gets in the way of "keep things constructive."
If someone is bothered by profanity in a way that makes them uncomfortable participating in an open-signup forum, they can choose to say so when it comes up. Pre-emptively codifying "correct" language is a little icky.