There is supposedly wireless-blocking paint. Only active blocking is illegal.

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?"

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.

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?

All you need is a building with walls, then BAM, Sprint is blocked.