Its not the law.
My guess is that some 3rd party offered the convenience store the scanner system for free or discounted in return for collecting data on his customers. The store owner likes it because he doesn't have to entrust his liquor and tobacco licenses to the poor math or judgement...
I've been doing some research. While I'm always game to try, it seems like some of the BCGs are a PITA to drill and tap.
Bear Creek Arsenal has a stripped side charging carrier for less than $40.