You are completely correct. With no black market...there would be no REASON for the violence. Consumers have the right to choose which places they buy product from.
For instance: I live in Warren right along the border of Detroit. Probably about 85% of my block smokes weed. When it was illegal, you would either have to drive to shady neighborhoods in Detroit to get it (supporting the drug black market) OR buy it off of a dealer from our neighborhood (usually getting yourself ripped off on money and also supporting the black market). The dealer thing was getting out of hand for a minute, they were EVERYWHERE, and every time you spent money at them you know your funds went right into the unsavory activities of the black market through trickle-down. Since they are unprofessional hooligans, they brought fights, the whole "hood" mentality, cops were constantly surveying the area giving us all that Big Brother uneasiness, there was a gun fight in front of my neighbor's house, etc.
Now most everyone on my block is legal medically to smoke, because nearly anyone can be. Everyone is less out to rip people off now. Everyone is happier and seems to have a better overall sense of pride, knowing they aren't going to get their shit raided any second and be made to feel like some giant felon. The drug dealers are ALL gone, except maybe a few who don't want to have to get real jobs. They're getting desperate to the point now where they're just straight LYING about their wares just so they can get a sale. But people still aren't buying it...why buy something off the streets when you can get BETTER for cheaper, legally, safer (you know it wasn't dusted or fucked with), and in a professional store setting?
I don't see the black market being in this neighborhood much longer...that's for DAMN sure.
Store settings also provide more alternatives than just smoking...brownies, cookies, rice crispy treats, COFFEE, tinctures (can be eaten or put on joints/bowl to enhance and help it smoke longer), keef tablets, hash (which used to be impossible to find in MI), you name it. So a varied market to cater to all sorts of people that is regulated, safe, earns us millions of dollars every year, encourages agriculture in Michigan (all hydro you get from these dispensaries are Michigan grown by reputable growers), etc...SOUNDS AWESOME!