An airport is private proprety they can have any kind of restriction they want to have.
Except that most airports aren't private property.
Hartsfield-Jackson and LAX is owned by the city.
BWI is owned by the state of Maryland.
Logan, JFK, and LaGuardia are owned by their local port authorities.
Dulles and Reagan are co-owned by Virginia and DC.
That is a small number that isn't most.
I think he was being lazy and didn't feel like going into every airport.
Busiest airports in the country:
Hartsfield-Jackson (Atlanta) - public
LAX (LA) - public
O'Hare (Chicago) - public
Dallas/Fort Worth - public
Denver International - public
JFK (NYC) - public
Phoenix Sky Harbor - public
McCarran Int'l (Las Vegas) - public
George Bush Intercontinental (Houston) - public
Charlotte/Douglas - public
Miami Int'l - public
San Francisco Int'l - public
Orlando Int'l - publc
Newark Liberty - public
Minneapolis-St. Paul - public
Detroit Metropolitan - public
Seattle-Tacoma Int'l - public
Philadelphia Int'l - public
Logan Int'l (Boston) - public
LaGuardia (NYC) - public
Washington-Dulles - public
BWI (Baltimore) - public
Salt Lake City It'l - public
Fort Lauderdale-Hollywood Int'l - public
Honolulu Int'l - public/military
General Mitchell Int'l (Milwaukee) - public
Tampa Int'l - public
Ronald Reagan (D.C.) - public
Chicago Midway - public
San Diego Int'l - public
I was going to keep going until I found a private one, but I'm getting tired. There's 30. If you want more, I can keep looking.