In any player-driven economy, especially with a large user base, in-game currency can flood the market, particularly from rewards, events, or frequent playing.
A tax on transactions acts as a currency sink, removing currency from the economy to control inflation, stabilize the market, and maintain the value of in-game currency.
Without a tax, the in-game economy would inflate, flipping would become exploitative, and game progression could be severely unbalanced. This would hurt both the competitive integrity and the financial sustainability of the game.
Keeping the incentive to purchase in-game currency is part of the equation, but helping to balance the economy plays a much larger role.