I have often wondered the very same thing. While I cannot give official advise, I think some sound judgement may dictate what to do. For example, fire ban when you enter and you ahve that huge rain storm. After a day is the earth still moist? What about the underbrush and downed limbs, trees, etc.? If you believe there isn't a risk of forest fire then I think a campfire is ok. Same goes for the second scenario you mentioned. If when you enter there is not a fire ban, but a ban is put into effect whilst you are enjoying the park, use some common sense. If its windy and dry, probably not the best idea to have a fire.

I think also if you have a fire and aren't allowed one and a ranger comes your way they could explain to you that a ban has been put into affect and check your permit to see when you entered, realizing there wasn't a ban at the time.

Basically, think.