YOU may not pay money, but I am sure there is some part of the game that earns the company that produced it money. In fact I've seen several of those "Free" online games that are designed to push as many of their customers into buying advantages in game. Personally, I'd prefer a flat monthly fee over someone being able to buy their way to being a better player.
Not saying Star Sonata is like that, as I have no experience, but many many online games that had potential end up going that route, and in some cases loose all business credibility because they sacrifice their morals and ethics.