I know, right? Do Milan or Inter fans celebrate in Turin like this when they win?
I was actually in Turin the day Inter became champions back when we were in Serie B. And yes, there were Inter fans celebrating on one of the main squares. Only a handful of them though.
If you go to Piazza del Duomo in Milano you will see a lot of the street vendors sell Juve merchandise (small amount, but still there) right next to the Inter and Milan stuff. Even near the Meazza I saw one or two of them that had a couple Juve shirts amongst everything else. Y'all are everywhere.
The street vendors in Turin sell Inter, Roma, Napoli, Milan, etc stuff as well. And similar to what you're saying, I saw other team's stuff being sold around our stadium as well. Only saw that once or twice though and it doesn't seem to be the case anymore these days.