Scubawasp
You have, I think, hit the nail on the head. When the TAF proves totally wrong and a diversion is needed, then most airfields are glad to have you safely on the ground.
When you didn't get a TAF or the weather did not improve as hoped for then its only right to pay the landing fee and accept the hospitality as though you had intended to be there, just like those standing around you.
Definition of a weather diversion (IMHO):
Unable to make destination and unable to return home, both due to unexpected change in weather.
Lack of hoped for change in weather should not count. Or should be a fine of double the landing fee for being too optimistic
