The anonymity that these car sharing services provide is already an issue. In my neighbourhood car2go are the most likely to be recklessly driving and harassing people in the streets. They should be tightening their policy not loosening it. I really believe there needs to be a standard for this (if one doesn't already exist).
It probably varies from city to city, but I always assumed that the reason car-sharing drivers are worse than others on average is just because they get less practice, are more likely to have learned to drive in a different country (with different traffic rules, layout of bike lanes, etc.) and perhaps don't know their way around as well (so they're experiencing a heavier cognitive load).
There is no reason to assume that people driving cars they don't own are as careful as driving their own cars. This makes a huge difference.