In Maine there are lots of early 20s workers that mostly seem to come from Balkan countries. I think those little tourist towns just don't have large enough populations to support the demand. Tourists probably outnumber actual locals. Places like NJ and Brooklyn though have plenty of 16-22 year olds who will work beach jobs.

I think it depends on what countries the owners have connections to. Random ice cream shop I visited in Florida a few years back was staffed by young Turkish women, my wife asked they said there on J1's. Before that we were in the Florida Keys - the local mosque was full of young Uzbek men that worked in the hotels, all on J1s.