For most frum Jews, America starting with the 20th century is our first experience in a capitalist system. In a market economy, consumers get to bend the business to their will. The bowling alley managers were reacting to the threat to take their business elsewhere.
If the skirt was a safety issue, they can address it through a waiver of liability. That they didn’t do so suggests violation of the statutory (and common law) duty of a merchant to serve all customers.