The Devious Design
The thing with this game is that, while you can download and play it for free, there is a plan worked into the system to tempt you to pay for certain things like special bonus items. This is mainly because the game is highly addictive, and as you gain experience and go up in level, you will find it a lot harder to play. Those amazing candy popping skills that you initially thought you had start to weaken when everything is moving so fast. At this point you get that tempting desire to buy credits and other bonus items. This is how this game rakes in money while remaining "free", and it is a marketing tactic that has worked remarkably well.