Most New Yorkers do not put any ketchup on their hot dogs.
The New Yorkers tradition for hot dogs is mustard and sauerkraut and sometimes onions or sweet relish on the hot dogs.
So while most New Yorkers do not use or put ketchup on their hot dogs some might.
Just because it's a tradition in New York to not use Ketchup on your hot dogs when you eat them doesn't mean that you cannot put Ketchup on your hot dogs.
I put Ketchup on my hot dogs when I was in New York although it's not the traditional way they do it.
I love my hot dogs with Ketchup and some may not.