Laws such as this have to remain in place to provide the authorities for a framework in order to be able to prosecute when someone has clearly misused information obtained. That is why there is a tolerance. Removing the law would be 'carte blanche'.
I agree with Capetonian. As a long-time and regular monitor of airband and other transmissions, I fully understand that what I am doing is illegal under current UK legislation, and I could not argue otherwise.
However it seems to be the norm that the authorities mostly turn a blind eye to this practice, as long as common sense is applied. My 'overhearing' a transmission between ATC and aircraft giving, for example, a clearance to a new height or heading, is pretty innocent and harms no-one. And of course I keep this to myself. I note that these types of routine transmissions are reproduced without hindrance, in such publications as "Flying the big jets" and several ATC guides.
However, hearing sensitive/confidential information and relaying this to all and sundry, well then I would expect to be hit with a big hammer and could have no defence. I think for these reasons (and to frustrate the criminal classes) the emergency services moved away from open FM transmissions to secure TETRA.
Regarding the comparison to the speed limit, it's the same. On a motorway 77mph is illegal, but you would not expect to be pulled over. However, 95mph is different, you are pushing it too far beyond the spirit of the law!