Here’s a discussion I found:
http://boards.straightdope.com/sdmb/showthread.php?t=561774
I would assume that arrest and confiscation may be troublesome enough to discourage above-board business activity, even if the Feds don’t prosecute. Cities have generally been successful in driving away legal Prop 215 dispensaries, when they’ve tried.
From what I understand about 19 is that the cities/counties will be able to decide whether they want the actual places of sale or not. Maybe he could go against that further will of the people, and maybe I’m just naive (and god knows it’s probably the case) but it would stun me if LA voters voted for Prop 19 and voted to allow the stores and he shut them down in the name of Federal law.
From what I gather about the towns who don’t want the medical dispensaries is that, well, it’s as simple as them not wanting them and it’s probably easier for the local authorities to get rid of them if the town/city passes a stature that says they don’t want them there. At least that’s how it works in Colorado.