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.
That's true for normal rental car companies, which are often very picky about damages. With car2go and DriveNow, they don't usually charge for damages anymore. It would be impossible to track. You're paying for the repair with the per-minute costs.
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.
a) My main worry is hurting someone else; I will be responsible for this either way.
b) If I do damage the rental, chances are I'll be held accountable at the end of the rental, and chances are the repair fees will be extortionate compared to fixing my own car at a shop of my choosing.