[vkk8]

Apparently it's not. I live in Finland and I've never had my internet connection cut off in a supermarket (or any other large building). This has happened to me sometimes in Southern Europe, though. I don't know if they boost the reception somehow in large buildings here or if the building materials are just somehow different.

[p_l]

It's mostly your random good luck and other people's random bad luck. A lot of building design choices can result in severe signal attenuation in microwave range, and not all buildings will have internal femtocells to cover that - or femtocells that accept your SIM.

Also, some buildings change over time in how they attenuate, especially freshly built ones where the walls are still "drying" can have close to 0 reception inside. When my parents built their current home, I had to keep an informal map of where the signal was strong enough to use GPRS (yay 7s ping in MUDs) and for voice we usually went outside.