My take has always been that if you enter under a fire ban, you are under a fire ban until you receive word (from another party or a ranger) that it has been lifted.  I wouldn't let a fire ban stop me from building a fire that I believed to be a matter of life/death, of course, but short of that, it's a luxury.  I'm not risking a fine or worse (such as starting the park ablaze) for a luxury.

If you enter and there is no fire ban, then you are allowed to have fires until you receive word (from another party or a ranger) that a ban has been put into effect.  At that time, I think you are morally, if not legally, obligated to follow the ban. 

Hey, I love a campfire as much as anyone.  I will have one every night if I am not under a fireban.  But I'm not interested in setting the forest ablaze and footing the bill for putting it out.