They want to keep the leverage it gives them, if they can't pin a real crime on a suspect then chances are they can get them for drugs then use that as leverage. I've known some truly disgusting cops who tried to get a guys ex wife to say he smoked weed around his kids when they arrested him for no actual reason and didn't want to be embarrassed
Holder is still a pos and "not challenging" is NOT the same as saying the DEA will stop overriding state laws and raiding dispensaries