There is supposedly wireless-blocking paint. Only active blocking is illegal.
If it's RF-absorbing paint (and not RF-reflecting), and it becomes more commercially available, will people start using it to paint other things they don't want seen by radio waves?
Until there is a hostage situation in such a store and some of the survivors complain that they tried to call the cops before the hostage-taker confiscated their phone, but couldn't because of the wireless-blocking paint...
You worry too much. There are plenty of existing buildings that naturally get little-to-no cell reception.
Not necessarily 'worrying' so much as just sayin'. And there is a difference between a building that naturally gets little-to-no cell reception and a building that was actively constructed to block it out.
I'll wager the vast majority of the calls that are made from within the store go something like "honey do you need anything, I saw X it reminded me of Y, do you want me to pick one up while I'm here?"
All you need is a building with walls, then BAM, Sprint is blocked.