And to add to IAHtraveler -- read the rental jacket (or previously agreed upon T&Cs if you're preferred). They reserve the right to assess damage found after the fact.

The "spin around the car" by the rental agent upon return of the vehicle is simple for convenience. They can, and will, assess damage after the vehicle is washed, etc. Read your terms & conditions of what you agreed to, before you complain. Just because you don't like it, doesnt make it wrong.