I can see how it's a tricky subject, because sometimes a law might have some bearing on the etiquette advice. The example the OP gave is a good one, I think--if someone says, "I brought some alcohol to a party and some was consumed. Is it proper to take the leftover home, or should I leave it with the hostess?" I've seen threads like this before and, IIRC, there's no hard and fast rule--posters usually just chime in saying what they would do and why. And someone could easily say, "Well, in that exact situation, I always leave it with the hostess, because in my area we're not allowed to carry open bottles of alcohol in the car." I think that's as valid a reason as any, and possibly a valuable piece of insight for other posters who might never have thought of that.
BUT--it does kind of peer down the slippery slope. If no one comments on that remark at all, it's probably okay; but if a tangent ensues with people saying, "Actually, you could put it in your trunk, the law just says the open alcohol can't be accessible to the driver," and then someone else says, "Well, the police shouldn't be pulling you over randomly and checking your car," and then a third person adds a story about how their sister-in-law got in trouble with the police for this very thing... That's when it starts to move away from the etiquette aspect into the dangerous legal territory, I think, and invites chastising or even locking.
So should the poster not have mentioned the (legal) reasoning behind their actions? Should it be phrased in a special way, with some kind of disclaimer? Or does the fault lie with the later posters who grab onto the legal comment and drag the thread away from pure etiquette? I am curious what you guys think.