Maybe I'm missing your point, but the reason they long haul is to increase the fare price. A cab from the airport to New York, New York is about 3 miles via local streets but about 6.5 miles via I-215. It's approximately $3 per mile, so they pick up an extra $10 by going on the freeway. Before I knew better, I was long hauled virtually every time I went to Vegas. Then, one honest cab driver took the correct route, and I realized that I had been duped my whole Vegas-going life, lol.
It's virtually never faster to take the highway to the strip unless there are really extraordinary circumstances. I don't know how many times the cab driver has recommended the highway to me because of "all the traffic" on local streets, but lo and behold when I insist, the traffic magically disappears.
The reason why you should say "shortest distance" rather than "faster" is that "shortest distance" is objective, while "faster" is up to some interpretation. The cab driver can claim they thought the highway would be faster in order to defend their actions. If you say "shortest distance" and they take the highway, that is clearly a violation. To be honest though, most of the time they don't ask and just do what they want unless you tell them upfront.
You are right that sometimes you want to take the highway, mainly if you are going downtown so you can avoid the strip, however NY/NY is not downtown.
Edit to add: anything south of NYNY (e.g. Mandalay Bay), highway may also be OK depending on the situation.