They try to get you to spend your money to unlock items faster, or get a one-time powerup. I hate games like this, such as Candy Crush, Asphalt 8, Dragonvale, Minion Rush, etc, just to name a couple. So many games are like this, which is sad. However, it is a completely different story to the game developers. If you read the Candy Crush on a newspaper, you will see that they can generate a billion dollars revenue quickly.
To those addicted to games like BTD, I really suggest you to try one-time pay games, such as Wii games and such. Thus, you are not wasting your money crazily. However, if you are non-addictive player like me, meaning you dont want to get all that stuff super quickly and get through the game, I guess that doesn't concern you then.