Those taking the stand that they will no longer shop at a store that wont sell guns to those under 21 better take a look at the rental policy of their favorite gun range. Most have a similar policy and always have. Most will rent rifles to those 18 to 21, but not handguns. Others have a no rental policy for under 21 for both unless they are in the military.
I don't shop for guns at Wall Mart or Dicks, because here in WA they really only carry bolt action hunting rifles, shotguns, and knives, and there seems to never be anyone behind the counter. As far as what they sale, that is their choice, I go other places anyway.
We need to choose our battles wisely and protecting the availability of ARs and the legal right to buy and own them needs to be out front, along with magazine capacity for rifles and handguns, or before long those stickers I see on my gun cases that say "Not Legal in California" are going to be a much longer label.