Yeah, some people will cheat. I've already written on this so won't do it again but I had some bad cases when I used to rent; the worst being an instructor (who also had a fake ATPL but that's another story).
It is possible to charge for airborne time and fuel separately, and catch the cheats (you compare the data downloaded from an engine monitor such as an EDM700 with the declared flight times, etc) but most rental planes don't have that equipment, and few firms will have the inclination to do it.
So we will continue to see people rev the thing up, engine cold, right in front of the hangar, and then taxi like crazy to the departure point.