"Free to play" has nothing to do with the cost you pay for the game. It refers to the monthly subscription fee model vs the non monthly subscription fee model. Modern Warfare is free to play. Lord of the Rings is free to play. Warhammer is free to play. Do you get it now?
You're right, it's not COMPLETELY free in that you do have to buy the game itself, but after that there is no sub fee, and "Free to play" in the MMO world simply means a lack of subscription fee.
As someone said, free to play does not denote free to own.