It seems in this case neither the passenger nor the cop was crystal clear about the law. Regardless.... Nothing actually happened. Nobody was wrongfully arrested. A cop said some stuff that seems legally questionable, but did not act on it. That's not "really weird" in my book.

The police officer caused undue delay and distress to the passenger of the vehicle. That's most definitely harassment.

Besides, the police shouldn't try to enforce particular laws until they are confident about their understanding of them.

Where in this article did the officer try to enforce whatever law it was he thought Uber was violating?

True, he didn't follow through, but he did bring it up in a way that suggested he was planning to enforce his flawed understanding of the law. My point is that he shouldn't even bring it up if he is uncertain.

He started to, then he realised he'd really fucked up.

The better argument, I think, is that it is also unrealistic to expect citizens to know all the laws.

The theoretical answer to this is that citizens are expected to know all the major laws that govern everyday activity, and are then expected to educate themselves when they engage in a particular activity or line of business that a reasonable person would expect might be specially regulated about more obscure laws that might apply to that activity. Officers, on the other hand, have no way of knowing what sorts of violations they are about to encounter when they go out on patrol. I don't claim, though, that this is always true of fair in practice -- I'm just relaying the conventional reply.

And I think some perspective is in order: the officer's information was, at most, a few weeks out of date, and he did not attempt to actually enforce any Uber-related law. Also note what I pointed out in my edit: the new law includes regulatory restrictions that the driver may well have been violating.