I agree in not asking for permission when the issue is unknown, ambiguous and unstated....
How about in a case where someone actually DIDN'T know, so they asked ? I mean, ... who better to ask, than the authorities themselves? Then wouldn't you assume that the answer you're being given,
is the law?
I know what you're driving at though: that .... later on, if someone found out something different, do they : a) go by their earlier "yes", or b) go by the subsequent info ?
Here's my answer to that: The mere fact that someone had first given you a "yes", means, in my eyes, that it's not as clear cut as you make it out to be. That is: it's a grey area type law open to interpretations and applications, as the needs-be-fit. So for example, if you're snooping around an obvious historic sensitive monument, vs in the middle of nowhere at a CCC spot.
VERY MUCH UNLIKE the non-grey area of speed limits, you compare this to. One is black and white (hence you don't ever find someone to "give you permission to break speed limits"). Versus this, which you can get a "yes" to. The mere fact of this tells you it is not at all as clear (like speed limits) as you say.