ifeldman,
Yes, I had the same concern. I think if you only use them in a tight spot, keep the car well waxed and the stick-ons clean it's not a problem.
I talked to one person at the company a few months ago, and they said we never had them stolen either. ( They are not tethered to the doors in any way. )
You just stick them on. Refrigerator magnet stuff. It's not going to pull the steel through the paint.
I think it comes the closest to an ideal solution, hardly a perfect one. They are thick as well as wide, which means even more protection. The further out they contact a 'wayward door', the better.