Why is there a double standard about sitting next to strangers?

Isn’t there a double standard to sitting by strangers? In my area they advertise heavy to get people to take public transportation but I know people who never want to get on a bus, even when they have to get to class or work and their car is down for repair; they would rather pay extra for a cab or walk. They say they don’t want to take the bus because “one of those crazy people” might come and sit next to them. Yet there are places and times when they do out of choice or necessity. If they are going to San Francisco and taking the BART train depending o the day they are forced to sit near or next to strangers. That also goes for the ball game or concerts, they have to sit by strangers and if you ever been on a trolley in San Fran you WILL be sitting by strangers. So what makes strangers on a trolley or at a concert more palatable than strangers on a public bus? Where is your “stranger seating line”? A jet plane, ferry boat, concert, roller coaster ride. What?