May have been fixed in the meantime?

I have SHOP_SPAWNING_PROBABILITY = 1.0E-4 //(must be between 0 and 1): 0 is no shops spawned in asteroid sectors, 1 is shop spawned in everyone (default: 8% -> 0.08), and I have a hard time finding any stick shops (as was intended by setting it that low)

Another possibility might be that a value of plain zero didn't work as intended, however with 0.0001, virtually no stick shops spawn anywhere.

Can't remember what version I was on when I modified that setting, but it can't have been very long after custom stations were implemented in 0.1595. It definitely seemed to be working in the latest stable 0.199.654, and long before that, too.
I have now set it specifically to 0.0 in 0.200.151, and haven't found any stick shops in a survey of about ten systems, besides the spawn shop at 2,2,2.

EDIT:
It may be worth noting that shops on custom stations do spawn, but I never expected them to be affected by that setting anyway, as there are other ways to prevent that (ie. don't have shop modules on custom station blueprints)