Legal isn't the question. Game attendees enter an agreement when they purchase a ticket. They can be escorted out if they fail to honor that agreement.
At the same time, the New York Yankees opened up a can of worms when they allowed and encouraged all the Black Lives MatterTM propaganda. This fan might have a lawyer take the Yankees organization to town.
But since the Yankees control the stadium during the game, can’t they put up pretty much any banner they want? Heck, they could allow BLM to put up a big banner and charge them $5 for the “advertisement”. But when someone brings in their own banner, a line is crossed.