Norwegian currently fly from two Caribbean destinations (PTP and FDF) to JFK, BOS, BWI and FLL with the 737-800 during the winter. Given that a potential BFS service would likely use the 738, I suppose it could make sense to operate to JFK. Yes, landing fees would be higher, but Norwegian already have the right aircraft and crew based out of JFK, and given the lack of direct competition that they'd face on the route, they'd be able to charge higher fares than on their potential EDI route.
Of course, this is purely hypothetical, and Stewart flights seem the most likely, but Norwegian's New York strategy doesn't seem to be that of a LCC: 6 European routes and 2 Caribbean routes out of JFK, and 1 European route out of EWR.