As I replied in another comment, where's the barter in your examples? I see gifts. If there is barter, yes, one must pay tax, or it's unethical.
Where did I say the cop was gaining financially? Why does that even matter?
Now you could say our whole system of taxation is unjust and thus skirting it is unethical. That's fine, but has nothing to do with get-out-of-jail-free cards, since, as you point out, the cop is not gaining financially.
The examples you gave are ultimately poor analogies because not only are the laws potentially broken different (taxation vs. speeding), but the officer is explicitly in a position of enforcing the law, making that scenario a conflict-of-interest.
A better analogy would be a government worker awarding a contract to a friend. Such an act is widely considered unethical, even if the friend charges the same and performs the same as another contract applicant. It's unethical, and a conflict of interest, because the worker is using their position of power for quid-pro-quo (a social benefit for the worker).
Of course, none of the above paragraph is true for a private corporation; it is beholden only to itself and can choose whatever chum it wants for contract work. Likewise, I could see an ethical argument in favor of a cop who pays from their own pocket the tickets of family members. Just not one who flouts his duty as a government officer for quid-pro-quo gains.
