It is actually a quite hard thing to understand, seeing as you didn't consider all factors of a free economy.
You would actually have to prove mathematically that having people sell their items and toricredits independently harms the total potential profit, which I am sure you cannot.
Some factors:

1.
Many users invest toricredits into boosters, items that get sold for USD and so forth to have a good start into the economy.
Furthermore certain services such as VIP don't get uncommonly sold for toricredits by users, who obviously will have to put real money on the line in exchange.
Common sense won't really get you far in this matter.


2.
There is a huge community around the market. Getting rid of it would enrage many people, and certainly make many leave as well.
As you probably know, there are many people who don't even play the game anymore but still can be found being active on the forums. Those are potential customers for nabi as well.
We do not want to deter people.