The militia is there to protect the rights of the people who used to own that land. You know, the ranchers/farmers that ruined the land for their purposes and were going bankrupt. The government bought their land for more than a regular citizen would have.

So those poor people were treated unfairly and the militia is standing up for them.

IIRC, they're also there because the BLM is bad because they have rules and charge for use of public lands and we all know that ranchers should get everything for free. 'murica!