Imagine the outrage of some F class pax who found out they paid F and are being put on a regional jet for the domestic US leg. JetBlue certainly would be a better product in this scenario!
I know many people who use the Icelandair flight to JFK and connect to B6 at JFK. Most of the year it is the only way to get down to S. Florida without overnighting enroute or driving from Sanford/Orlando.