This is the response I received from the customer service rep: (this is off their FAQ)
What can be mailed at the Media Mail rate?
Media Mail can not contain advertising, except eligible books may contain incidental announcements of books.
Books
16-millimeter or narrower width films (must be positive prints in final form for viewing, and catalogs of such films of 24 pages or more, at least 22 of which are printed).
Note: Films and film catalogs sent to or from commercial theaters do not qualify for the Media Mail rate.
Printed music, whether in bound or sheet form
Printed objective test materials and their accessories used by or on behalf of educational institutions.
Video recordings, player piano rolls, and sound recordings (records, tape cassettes, CDs, DVDs, etc). Including incidental announcements of recordings and guides or scripts prepared solely for use with such recordings.
Playscripts and manuscripts for books, periodicals, and music.
Printed educational reference charts designed to instruct or train individuals for improving or developing their capabilities. Examples: maps, tables of mathematical or scientific equations, periodic table of elements, etc.
Loose-leaf pages and their binders consisting of medical information for distribution to doctors, hospitals, medical schools, and medical students.
Computer-readable media containing prerecorded information and guides or scripts prepared solely for use with such media.
Notice that it doesn't say anything about anything needing to be educational, but I guess they can restrict video games since they aren't simple recordings or something. Still the whole situation seems like bull crap to me. I also received a similar explanation in my PO Box today from the PO clerk, and according to that if people want to know a book needs to be at least 8 pages to be considered a book.