Well it's like being locked in though, if you leave you can't come back. And I didn't at any point say I was forced to buy a shirt, I am merely pointing out that football fans DO buy shirts and even go as far as to have their favorite member of staff's name printed on them.
Your analogy of...
I don't willingly go to a pub, pay a large sum of money to get in, get locked in for the duration of the opening hours, and cheer the bar staff and buy the same Brewery branded T Shirts as they are wearing as they go about their job though.
That's the difference here.