Usually you can only get ZEDs for each segment you take. I think some airlines have deeper reciprocity agreements (I could swear I heard something about AS and HA giving each other free flights). The reason that we suddenly had the ability to get US benefits is because we work for SkyWest, which does flights for UA, DL, and AS already, and when it started a contract with US, they were generous enough to offer full benefits to all employees, even if we don't touch the US product. UA does the same thing (though there is a yearly enrollment fee for non UA-dedicated employees). DL and AS only gives full benefits to those that actually work on those flights, so because I work at a UA-only station, I can only get free DL and AS flights operated by SkyWest.