Actually, your suggestion reminds me of the DRM already being used by eReader (formerly PalmReader). There is no limitation to the actual device, or the number of them. You download an encrypted file, access it on any machine that can run eReader, but rather than a PK, you need to use your credit card number the first time to decrypt the info. Obviously, this creates a disincentive to distributing the file.
Still limits you to the eReader software, but since that is available on a variety of platforms, the limitation is minor,
if you use just those platforms.
My problem with your suggestion is that it assumes we could get all the vendors to agree on one standard format, with one standard way of locking the file if the vendor insisted. The same idea was essentially behind the OpenReader project, and if you have never heard of OpenReader, that should tell you how well they did. Sadly, for a variety of reasons, the content owners refuse to standardize in any way. Even a series of books, from one author and one publisher, may vary from one book to another as to which formats they make available.
Sounds like a simple oversight, but then they refuse to correct it... That goes beyond a standards issue. I cannot decide if it is outright sabotage or simply idiocy.
Jack