Zahavasdad – Who is supposed to ban the minyan? The Eida Chareidis of Citi Field? The minyan is “allowed” by the security guards. I am willing to bet that if you ask an Rov where you should daven, game vs Shul, they would say Shul.
I have davened at Busch Stadium in St Louis, but so what? Doesn’t make it right or wrong. And does not have anything to do with wearing a Yarmulka either!
The fact that Jews make a minyan there just shows that they want to have their “Gashmius” while not ignoring their “Ruchniyus”. It just eliminates the issue of missing minyan b/c of a game.
But the Minyan does not make the game/stadium into a “Frum” outing/event at all.