When Delta started its Latin America expansion out of Hartsfield, they arranged for a sterile "in-transit" lounge as they were targetting Europe-Latin America passengers (apparently MIA gets a lot of these...AA fills up a 767 MAD-MIA most of which continue onto latin am). I think you are escorted from the arrival gate to a sterile room where you are forced to sit and wait. At departure time for your next flight you are escorted through the concourse right onto the plane, like a prisoner. For free access to the terminal facilities you have to clear immigration. I wouldn't be surprised if DFW and MIA have something similar (connecting a lot of in-transit pax). I wouldn't expect JFK to have that many in-transit pax, and if they have such a set up you'd still probably want to clear immigration to use the lounge.

As a disclaimer, this ATL process might be COMPLETELY wrong. It was explained to me by a Delta gate agent in ATL who didn't actually work int'l flights, so it might not be accurate.