Growing up in NYC and later on Long Island, I would say that I have never seen the default to put butter on a sandwich. I suppose you could ask, but I am not sure all delis would have it readily available and you would most likely get a raised eyebrow for the request. I did not know this when I went to the UK, and I found butter made the sandwich way too sweet and a little greasy for my taste. Granted, I am also pretty much a condiment hater on sandwiches in general, so I am forever having to ask for them sans mayo, which to my taste buds is far far worse than a small slick of butter ever could be. Don't even ask about mustard...
For the record, I have never had a sandwich fall apart on me due to the lack of condiment of any sort- perhaps our deli meats are different?