Wow, that's strange. I have never owned a case that scratched my iphone. Had the bumper on for over a month and not one single scratch. Had a cheap Kroo case on my 3GS and not one single scratch when I went to sell it. I think it just depends on the owner and how they treat their phones.
I don't think it depends on the owner and how they treat their phone. I've seen cases, particularly Incase slider-style ones, to do some pretty serious damage to iPhones. And this is to folks who take meticulous care of their phone, myself being one of them. It is most often caused by small pieces of debris that find there way under the case.
You can be as OCD as they come and not prevent that damage from occurring. And sometimes it's the design of the case itself. As cool as the Power Support Air Jacket was for the 3G/3GS, it was murder on the bezel when you would take it on or off. Personally, I think that is the biggest culprit- removing and putting the case back on. And that can be pretty hard to avoid if you want to use a dock or an older iPhone cable and most 3rd party cables/car chargers when you have an Apple Bumper.
I have a bumper on my 4, and it's definitely facilitated damage being to the steel; on the power button there is what looks like a little chip and to the left of the headphone jack there's scuff marks.
All cases have the potential to facilitate damage. I'm definitely disappointed to see it, but not surprised.