Earlier quoted context omitted.
Are they rare in the US? Every stop I use regularly (in Chicago) has shops in it. The only ones I can think of off the top of my head that don't are ones where there isn't space or where businesses have left on their own volition.
I'm sure someone will show up with a counterexample, but DC doesn't have any that I've ever seen. I feel like they'd make a killing if they just stepped up their vending machine game to be more like Japan, though eating and drinking on the metro is supposed to be banned in DC, so I guess that would send mixed messages.
And it most certainly doesn't make any sense placing those stores on stations that are outside of the Beltway (there's simply not enough demand there, and stations that are farther out are almost deserted by the standards of EU cities)