If you've ever dealt with an officer of the law in an adversarial situation where you believe you've done nothing wrong, it can be an unnerving or even terrifying experience. The power imbalance is immediately and palpably felt (what I think the author was trying to convey by mentioning the clearly-visible gun). If the officer had wanted to arrest the citizen (who, if he was doing something wrong, was doing it in good faith with the belief and understanding that his actions were not only legal but encouraged by the governor -- an unnaturally well-informed citizen for being a temporary visitor to the state), he would have been well within his rights to do so.
Is this some rogue cop enacting some personal vendetta against alternative hirecar arrangements? Probably not. But did the officer act unprofessionally? Entirely. Not answering the questions, throwing the license back at the passenger, offering a free ride to the airport (which may very well be inciting breach of contract)...and what will happen to him? Possibly a slap on the wrist.
The only way the police can justify the power they are given is by wielding it appropriately and heavily penalizing its abuses. I don't expect the former to happen 100% of the time -- we are fallible humans -- but when it doesn't, I do expect the latter. And I rarely see it.