Being a child is all about boundaries. And boundaries are inevitably going to be set before a child can learn and understand why they exist in the first place.
Of course it's hypocritical to not let your child cuss when you do it "all the time". But in this case, that's part of the lesson. The fact that children aren't allowed to do it is the point: there's things you just don't do as a kid, and cussing is one of them. It's much more hypocritical when your own kid uses a bad word and you're all, "Now where did you hear that?!" Because 99% of the time the kid heard it from you, dumbass.
Kids must also learn that there's a time and place for everything. Letting them do everything freely is a recipe for disaster. If they learn that something mustn't be done, they'll learn to question if something is proper and acceptable before they do it. Don't put up clear boundaries for your kid, and what you get is Twitter.