Why can’t you get a slice of pizza at John’s on Bleecker or Patsy’s? Allegedly because of Al Capone:
In his 1981 book on the mob called Vicious Circles: The Mafia in the Marketplace, the late Jonathan Kwitny detailed how Al Capone — who owned a string of dairy farms near Fond Du Lac, Wisconsin — forced New York pizzerias to use his rubbery mob cheese, so different from the real mozzarella produced here in New York City since the first immigrants from Naples arrived in Brooklyn around 1900.
As the story goes, the only places permitted to use good mozzarella made locally were the old-fashioned pizza parlors like Lombardi’s, Patsy’s, and John’s, who could continue doing so only if they promised to never serve slices. According to Kwitny, this is why John’s Pizzeria on Bleecker Street still has the warning “No Slices” on its awning today.