> I'm unaware of any baseball park or hotdog stand where top-cut buns are sold.

Baseball Park: Fenway[1]

Hotdog Stand: Not uncommon in Rhode Island (and probably other parts of New England). I've seen Saugys[2] sold from food trucks in side-cut buns but it's the exception rather than the rule.

1: https://newengland.com/yankee/history/fenway-franks/

2: https://saugys.com/