The BLM can enter into a memorandum of understanding (MOU) with government agencies, nonprofit organizations, private individuals, or any person or entity. As stated on their website "unless authorized by the land owner" the BLM can have jurisdiction over private lands.
Rancho Las Flores is known to purchase private land that they intend to exchange with the BLM for private land that they want somewhere else. It makes sense for them to enter into MOU's to lessen their management costs until the exchange occurs.
http://www.ca.blm.gov/barstow/jflats.html
"Private land owners, legal grazing allotments, and mining operations with Operations Plans on file with the BLM within the Juniper Flats area continue to have rights of access. It is not the intent of BLM to affect private rights or interests in the area. Further, this does not imply any BLM jurisdiction over private lands within the area, unless authorized by the land owner."
In addition, the law clearly states that in order to press charges for trespassing, the private land must be clearly marked as private. Otherwise, how would anyone know they are trespassing?