If you live in a city, or are in a city, you are not allowed to wear a shirt that bears the name of that city.
So, if you go visit a city, and you buy a t-shirt that bears the name of that city, you are only allowed to wear that shirt after you return to your own city.
Exception, sort of: You are allowed to wear t-shirts that bear the name of sports teams from your own city. I mean, I wouldn't. But you can.