From the Audubon Society of Portland:

The occupation of Malheur by armed, out of state militia groups puts one of America’s most important wildlife refuges at risk. It violates the most basic principles of the Public Trust Doctrine and holds hostage public lands and public resources to serve the very narrow political agenda of the occupiers. The occupiers have used the flimsiest of pretexts to justify their actions—the conviction of two local ranchers in a case involving arson and poaching on public lands. Notably, neither the local community or the individuals convicted have requested or endorsed the occupation or the assistance of militia groups.

Apparently a set of keys was found outside by the "militia" thus letting them inside the small, unprotected bird sanctuary.  This ingenious hostile takeover is made even more impressive since while they are claiming to have 150 occupiers, journalists on scene are reporting only 6-15.

The real problem is that these terrorists' repeated claims to federal land are damaging the environment.  From the Center for Biological Diversity on the 2014 Cliven Bundy illegal grazing debacle in Nevada:

"The Gold Butte area south of Mesquite is officially designated as critical habitat for the tortoise – an area essential for its long term survival. But the BLM continues to allow grazing by trespass cattle.
... 
Despite having no legal right to do so, cattle from Bundy's ranch have continued to graze throughout the Gold Butte area, competing with tortoises for food, hindering the ability of plants to recover from extensive wildfires, trampling rare plants, damaging ancient American Indian cultural sites and threatening the safety of recreationists."

The United States needs to step up on federal land management so this type of terroristic action doesn't become seen as a viable option.  These armed sovereignists want to become martyrs or messiahs, but what they really need is to be imprisoned and fined.