just a couple clarification
1) immunity is only for lower ranks. People that it's not even good at aiming yet.
2) immunity is only when you respawn after dead. It's not when you are on the airport, so you don't get immunity just by landing and reloading.
The idea is very simple, you don't want someone that just died 10 seconds ago, to be able to die again and again and again... 10 times in a minute just because someone is camping next to the airport. That's all.