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.
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)
Which ones do you use that has shops?
- 53rd Street Metra (bicycle/coffee shop)
- Randolph Street Metra Station (~8 or 9 shops of various kinds)
- Merchandise Mart Brown Line (attached to a mall)
- Clark & Lake CTA in the Loop (entered via 2 different shopping centers or the street)
- Randolph/Wabash CTA in the Loop. No shopping at this one as it is an old, small elevated stop. That said, in the little space it does have available there is vending so it has been commercialized.
The red/blue lines are good examples of what I mean by not having space. Where would you put shops that would generate enough revenue for the work of putting them in to be valuable? The 95th street station used to have a newstand in it but doesn't anymore, I assume because they couldn't find tenants.
Obviously there is also the pedway at Lake St, and merchmart, etc but that's not really what we're talking about.
I'm from Chicago, and I was a bit surprised when I went to NYC for the first time and saw stores on the opposite side of the turnstile. I know this is quite common throughout the world, but it is relatively uncommon in the United States.
As far as I know there are not any stores in Chicago that can only be accessed by paying a fare.
To answer the question of the 5 stops I use most 4 of them have shops. None of them are on the platforms, but the platforms are very small. All 4 of those that have shops have been completely remodeled in the last 15 years to make the shops more appealing or to add shops that didn't exist.