It’s simple. Google Drive is a great product but does not allow you to share files and folders with non-Google emails. The recipient is notified of insufficient privileges or that he/she needs to create an account. This is obviously a business decision aimed to recruit more users but to me greatly hinders the service.
If you have an opinion one way or another please add it to the discussion at the link below. My main goal is to get Google’s official position on the matter.