Um… this is a matter of excessive authoritarian zeal, personal opinion, or dollar signs (imagine all the people driving through their town that they could ticket for having these). Whichever it is, these people should be publicly slapped down for abusing the law.
Leaving aside the obvious First Amendment issue, consider this: if testicles are obscene, then the owners of any male animal — including unsterilized dogs and horses — should be forced to cover them, or must be fined for displaying them as well. If they’re not, then having fake ones displayed on your vehicle must be permitted. Only finding them obscene in certain instances is selective enforcement, which historically “is recognized as a sign of tyranny, and an abuse of power,” according to Wikipedia.
(For the record, I find truck nuts to be uncouth — though amusing — and I wouldn’t allow them on a vehicle I own. Nevertheless, my argument stands.)