Let us say I have accumulated 50,000 air miles and use it on a flight on Alaska Air from Vancouver to Anchorage. Now , is it correct that just by taking the flight itself, I will earn back 'all' those 50 000 miles. It seems like the flight is free in that case.
Flight from Vancouver to Anchorage is approximately 1571 miles. Alaskan Airlines may charge you 50,000 miles to book a flight but you only earn back 1 mile for every 1 mile you fly. So in face it may take you 50,000 miles in flying in order to earn 50,000 miles or by opening a new credit card with Alaska Air.
It is a marketing scheme where you earn miles by opening a credit card and use it to book flights or joining their frequent flyer program.
Hope you check and understand difference between air miles earn and use policy.