Making the new “fees” (Note, not fines - fees) applicable to all would hurt tourism. Mustn’t do that. Major campaign contributors wouldn’t like it.
This is an implicit admission that the new tax (let's call it by its right name; the purpose was to squeeze more money out of the citizens, not to enforce traffic laws) will bite perfectly ordinary drivers -- if it only affected truly bad ones, then keeping them away would be a net benefit (the loss of a few tourist dollars would be more than offset by reduced accident cost).
There are quite a few places where the speed limit drops by 10-20 MPH with grossly insufficient warning. Given that drivers on major roads need to exceed posted limits by 5-10 MPH in order to safely keep up with traffic, that means they will inevitably be 20-30 MPH over when they first hit the speed trap.