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That's a pretty unsubstantiated assumption. Could it simply be that cell signals have trouble penetrating the store walls?

I think it might be the fact that it's a rather large concrete box with very few areas that signal can pass through.

There are building materials that naturally block/reflect/absorb mobile frequencies which is perfectly legal.

That is different from transmitting a signal to jam which is highly illegal.

One other trick is to advertise as a cell and accept connections, then just black-hole them.

Full signal strength but no calls.

Edit: may be illegal in your jurisdiction if you require a license to transmit on cell frequencies.