Last time I was in Seattle, I did see a couple of cannabis stores from the outside. I didn’t see a whole lot of them (they’re not as ubiquitous as this ad implies), and the couple I saw had no window displays of any merchandise. What they did have was someone at the door checking the ID of everyone who entered. I discovered later that the Washington state law requires an ID check at the door and again at the register to ensure no one under the age of 21 enters the store or makes any purchases. It appears the proposed Massachusetts law is modeled the same way.
It’s difficult to see anything with this arrangement that would suggest a problem with minors getting easier access to marijuana. It’s still easier for them to get liquor, which is what the alcohol industry wants, anyway.