First, Miller Park is 71% owned by the government (the Southeast Wisconsin Professional Baseball Park District), so it is a prime candidate for my arguments that through joint participation, the team becomes, at least for ballpark purposes, a state actor subject to the First Amendment and its limitations.
Second, note the vacuousness of the Brewers' statement, which toes the common line on ballpark speech:
We welcome the opportunity for fans to express their opinions. The only circumstances that would warrant us intervening is if someone were to display a message or item that would be considered offensive to other fans.But every message potentially could be considered offensive to other fans. A friend of Ryan Braun or a member of his family easily would be offended by that shirt. Of course, that is not what the Brewers mean--that mean what they--as the governing authority--would consider offensive to other fans. But we don't allow the governing authority (when subject to the First Amendment) to decide what speech is OK and what is offensive.