I believe dispensaries have to verify there is no dangerous mold or chemicals in the weed, especially since "medical" patients are generally suffering from compromised immune systems, common mold that may not harm a normal person, could kill a patient thats been diagnosed with any chronic diseases that affect the immune system.
Well, I'm not sure on the legality of the quality control that they have to abide by given its state of infancy, but I know its an issue that they would eventually have to keep in check if it was ever legalized in a large scale for recreational use, it would only be a matter of time before someone grew their weed with mold, sold it to a careless dispensary who didn't test it properly and end up with a lawsuit from selling cannabis deemed not safe or potentially harmful to the patients they service.