We have a free-landing policy here for genuine emergency weather divs, and in my experience it is bl**dy obvious if its genuine or not as the pilot(s) are usually so happy to be on the ground they couldnt give a flying !!!! about any landing fee; when they are not charged it comes as a pleasant surpirse.
I think the distinguishing ferature is whether the div was "planned" or not-how you quantify "planned" is not so easy. But like I said above its always been fairly obvious to me the genuine cases and am not aware of anyone abusing the system here- if people do starting abusing it then wo-betide GA in this country.