Or maybe if you do not have a specified fine, then in the heads of people there is still a “virtual fine” associated with being late, but it is not an exact number, but a wide range, and people tend to predict all kinds of bad things that would happen (but 99% of times it wont happen), so the fear of an unknown punishment is higher than a known one.
Intuitively the present value of punishment is amount of punishment x probability of getting it, and if the second term is nil then so is the punishment; but according to many studies I believe the amount of punishment is much less important and being caught is what matters.
