The thing is, you don't generally get to know the context or the intent. You can't have a discussion with the sign, nor can it lecture you. A rule against starting a fire might be because the land owner doesn't like burnt patches on their meadows, or it might be because the vegetation is super dry and if you set fire to it you kill not only yourself but also all the surrounding villages, or something in between.

You could argue that the sign should include enough context to convince the reader to follow its instructions, but (a) you end up with signs with tons of writing in tiny font that everyone just ignores because TLDR (and yes, these do actually happen quite frequently in parks around here), and (b) if there is some combination of letters you can put on a sign that works to stop people lighting fires, the meadow guy will put that on his sign because he doesn't want fires and those syllables work. So you've just pushed the problem one level back, but the real question remains the same: do you risk doing the thing you want to, or do you respect the sign?