1. Who cares how full the tank is when you get it, as long as you aren't required to return it full? (OK, ignoring ridiculously empty tanks here.) When the tank is full, there is a lot more free gas you are giving the rental company because the gauge usually doesn't begin to drop until quite a bit of fuel has been used.

2. A contributing reason for Enterprise not having full tanks could be that they use the cars to pick up customers. The cars don't just sit around on the lot waiting to be rented.