So?
Which came first the chicken or the egg?
They go to get the food because they have the munchies, or they get free pot and then get the munchies and get more food.
How is this legal, even in California? 
California dispensary sparks food drive with free pot
Express-News -
SOQUEL, Calif. (AP) — A California medical marijuana dispensary has raked in food donations with a unique offer: free pot.
The  Granny Purps dispensary in Soquel (soh-KEL'), about 60 miles southeast  of San Francisco, offered a complimentary marijuana cigarette for every  four cans of food a patient brought in this holiday season. Each patient  was limited to a maximum of three cigarettes a day.
The 
Santa Cruz Sentinel  reports that the dispensary took in 11,000 pounds of food and handed  out 2,000 marijuana cigarettes between November and Christmas Eve, when  the promotion ended.