I don't know about the US, but in Germany if you return your car with scratches or dents that haven't been there before they charge you for that, so I wouldn't drive them less carefully than my own.
A few years ago, they even stopped asking customers at the start of the rental whether there were any “new” damages with the car, since it's impossible to verify that with dozens of damages, and people would just press “No”. So now these cars all have small damages that nobody cares about, which accumulate until they get repaired.