"The point of libraries is to make content freely available for the common good, I thought, so these restrictions are a little weird. Physical library cards don't require a certain type of wallet; why should the electronic ones only work on Windows?"
Read Public Libraries, Private DRM. The discussion chain is interesting too!
As for my 2 cents ... the problem isn't a DRM one, it's that neither of the leading library downloadable audiobook vendors, OverDrive & NetLibrary/Recorded Books seems to offer a solution that plays well with the most popular player. To me it just doesn't make smart business sense to build a service based upon only 28% of the MP3 player market share.
But then again, I'm not the vendor. I'm just a library staff member who frequently has to reply to patron inquries and frustrations about the service. :)