Airborne units may only be dropped within 8 hexes of a supplied friendly unit.
You can't just drop units as far west as you want. It has to be coordinated with your front line units. I don't like running away as SHC because it's boring. Instead I counter-attack and I'm willing to sacrifice a lot of units. The airborne drop is a benefit of this strategy. I'm sure I will pay the price later.
Overall, I find this very realistic and interesting. Why is it not possible for 300 men to land behind enemy lines to destroy the rail in the hex? Your screenshot shows the perfect way to defend against it. Use airbases and HQs to defend the rail line. You just have to make sure that the front is not moving too far west.