I am pretty certain that would not hold up in court: because they do very specific curation of their App Store (including reading the descriptions for content and vetoing certain forms of text), they are legally liable for all of the content displayed in their catalog.

In fact, due to the way the American legal system works (with our concept of "joint and several liability"), Apple would probably end up paying any and all trademark damages, and would then have to go after the developer to recover as much as they could themselves.

Nope. When I first submitted SharePics the logo had a white space at the bottom. They rejected it because they said it looked like a Polaroid. Apple needs to be more consistent, either curate the hell out of it, or don't. Inconsistent application of the rules make it more confusing for naive users.

Not only naive users, it makes it pretty hard to get a feeling for what's acceptable even for seasoned developers with good intentions. (Volume button for camera, anyone?)

Apple does respond to complaints from trademark holders. It's not Apple's job to determine trademark infringement; it's the job of the trademark holder to protect their marks. When Microsoft files a complaint, Apple will take down the offending app, assuming the violation can be proven. The burden of proof (as in the courts) is on the trademark holder, not the seller.

They usually do catch or question or flag apps that can't provide license to use trademarks. We publish apps using well known apps and they sometimes get rejected until we can prove we have permission by the mark holders.