I think you are correct on this, this is indeed a civil matter, it's called voluntary parting, in other cases that made it to this site where people have tried to report to the police that the camera they rented out or something was never returned were told it's not theft. I think what happens is that they report the vehicles to the police as stolen, as they would if they found a car missing from the lot, instead of telling them someone did not return it.

This varies. Some state laws specifically criminalize the failure to return rental cars in plain language. For example: https://law.justia.com/codes/nevada/2010/title15/chapter205/...

I'm not 100% sure, but believe this is the case in many states.

As to why some states criminalize it... I don't know, too many years of voting for tough-on-crime politicians?

I'd love to see which states consider it civil-only.

In every one I've heard of, it can (and eventually will, for varying values of 'eventually') get you arrested for grand theft auto.

I don't disagree. Let me slightly backtrack on my previous statement: I think some states explicitly spell out penalties for failure to return a rental car, while in others, it may be up to the specific circumstances to determine whether a crime has taken place or whether it is a civil disagreement or misunderstanding on terms, etc.

I remember a recent rental car contract of mine that pointed out these specific state laws. e.g.: https://law.justia.com/codes/nevada/2010/title15/chapter205/...

Because then people without sufficient assets to go after would basically be unable to rent a car.