No not bribes in any sort of monetary form, that would probably create too large of an outcry if it happened and was provable.
People may very well create the associated content for those books but unless the frame work for it is in the bill, good luck getting such extra content adopted by professors/universities/community colleges. Having pre-made tests/quizzes/homework available would be great but there is also the extra work of putting it in a format easily incorporated into existing front end systems. How easy would it be to import something like that into say, Blackboard?
Not to mention needing proper outside verification that the material is accurate and helpful.
Where as all of the above is already provided by publishers of existing text books, it's a long steep uphill battle to get these books adopted by professors.