With regards to laws....all the poster has to do is follow the laws of the State from which they live. Otherwise, when I travel from Ohio to Iowa, my 10 year old would be required to go back into a booster seat, because Ohio Law is age 11 I believe.
The police know what cars are rental vehicles and which ones are not. Again, they are only required to follow the laws of the state in which they live for booster/carseat.
The majority, if not all, states require for children age 1-4 to be in an approved carseat/booster seat, and those over the age of 4 to be in a booster. There are weight/height requirements, but it would be to involved.
Ah, no, actually, the laws are strictly enforced here in FL and the enforce only FL laws. We were trained to point out the posted FL laws to renters. The police only know if it is a rental if they run the plates or look at the rental agreement. In fact, we were not allowed to let any renter leave unless the child was properly restrained. Let me tell you, that could get touchy.