There's a lot more that goes into it, but first off, they don't even normally have saver level space on the TLV flight for their own members either, whereas they do on the LHR flights so that's why you'll see them on AF.
Regarding why they don't release saver, one reason is probably that they have 1 frequency to TLV, while to LHR there are so many flights from NYC every day across several airlines. More capacity means more seats for sale which means more potential for saver space to fill them up. Again this is obviously a huge oversimplification as there are many factors that go into saver space and it seems to vary for every airline, but that's the basic picture.