JetBlue has nonrev agreements with a lot of carriers, but few of the majors. The reason seems to be related to ticketing, since JetBlue doesn't list on most reservation systems. It also doesn't issue paper tickets, so they'd have to be handwritten. That's a problem with the majors, not at JetBlue's end. I can only assume that once demand is high enough, the majors will make accommodations for ticketless travel or come up with a service charge arrangement.