My first thought was they inadvertantly stumbled on a grow field. But with pot basically being legal out there how secretive do you need your grow field to be?
There are still many illegal operations happening in the Sierras. You do need a license for growing more than a few plants.
The legal grow operations are few and far between as they are so regulated that most farmers have reverted to illegal operations.
They sell if for cheaper than the state sanctioned, taxed variety.
"Legalization" has increased the black market.