Some of it goes back to railroad land grants when the federal government was trying to encourage settlement of the west in the 1800s.
http://en.wikipedia.org/wiki/Checkerboarding_(land)
Land out west was plentiful and was not at a premium until recently, as the population grows and more people want to use public land and it becomes more and more crowded, it would not surprise me to see the government (state, federal, or local) work to provide access to bigger sections of public land by negotiating easements (backed up by the threat of eminent domain). Historically, that has happened in the east more with water access.