Since noobs will get killed in any plane they fly, why not let the entire planeset in WWI be free?
From a marketing point of view you are correct. However I was envisioning 3 basic goals:
1. Increased marketing for HTC by offering unlimited-time free-play in their non-premier arena.
2. Serve the needs of the arena by limiting them to role that are benifitial to the the arena but are less likely to be chosen by paying customers.
3. Provide critical-mass forr the WWI arena and targets for the paying customer.
So allowing them the full planeset wouldn't server goal 2, IMHO. The arena would still be nothing but DR.I's for the most part. I think getting the other, less seen planes in the air, giving the paying player more variety to shoot at, is a good way for them to pay off their free passage. If they get to the point they really want to unlock those other aircraft, they can convert to pay.

I can see the free-player be switched to what ever side serves the arenas needs. You don't get to pick a country when you launch. You are assigned a country based by the sever to balance numbers.
Maybe you don't even pick a field, or even a plane. Maybe you only have a
"Volunteer" button and when you click it, the server places you in the country, the field, and the plane it thinks is needed to balance game-play.
Or maybe it places you in a manned ack at a field under attack.
Or in the unfilled gunner position of a bomber or recon plane.
You get to play for free, but you don't get to choose. The server assigns you where ever is needed to improve game-play.
Regards,
Wab