This is fraud, plain and simple.

If the airline caught you and decided to prosecute you (which they probably would not) you would be convicted and sentenced. Fact of the matter is the airline wouldn't bother prosecuting (they'll just shut down your account and be done with it), but that does not change the fact that it is definitely illegal.

You can argue the airlines deserve what they're getting and so on, but that does not change the fact that it is fraud.