I’m one of those, and AFAICT, getting the shot is a question of availability and demand. Our CVS here in Massachusetts had the vaccine on hand, we asked for a booster, and we got it. The same was true for lots of people at the school where my wife works.
In Massachusetts, 68% of the population has been fully vaccinated, and 78% has gotten one shot. Since the vaccine has a limited shelf life, I imagine the thinking is that if there’s enough on hand, give it to anyone who asks, rather than let it expire.
It could be that where the vaccine is in shorter supply, dispensaries are being more restrictive. But my advice to everyone is to try to sign up for a booster at your local pharmacy (without lying or dissembling, of course) and see if you can get it. The worst that can happen is that they say no. You won’t be arrested.