Or maybe the PC was in the back of the shop, in some non-public area that a bunch of random, inquisitive people (regardless of age) weren't welcome to visit.

Maybe the area was just trashed or otherwise unseemly, and would leave a bad impression. Maybe the shopkeeper smoked weed back there and didn't want to expose kids to Bertha, his prized six-foot bong. Who knows.

When I've worked in shops that sold electronics with a retail storefront, we tried to keep the showroom looking very nice and uncluttered. The back rooms where we repaired stuff and tinkered around with gear were very different: These were utilitarian spaces that regular customers weren't often invited to see.

Nothing particularly untoward ever went on back there, but still: There's reasons why shops (of all sizes) have interior walls. :)