Yes, a stadium is most certainly regarded as a "public venue" and neither O'Connor nor Thomas disputed this in their dissents. They clearly and explicitly agreed with it.
Yankee Stadium and Shea Stadium in New York City are clearly "public venues," since the City of New York is the legal owner of both facilities. A different situation could occur (and does occur more frequently these days, as these venues get more expensive to build) in other places. If a corporate entity such as the Ballpark at Arlington is listed on the property title as the owner then the stadium is no more a "public venue" than the Wal-Mart across the street.