There is no reason why textbooks can't be DRM free.
It is just one of many business decisions an author/publisher makes.
Some textbook sources not only go DRM free, they *are* free. Just check online for "open source textbook":
https://en.m.wikipedia.org/wiki/Open_textbook
http://open.umn.edu/opentextbooks/
http://www.teachthought.com/technolo...rce-textbooks/