The online casinos have wagering requirements for their no deposit bonuses. You'll need to wager the bonus a specific number of times on the video slots in order to withdraw any of your winnings from the casino. Also, their will be restrictions on the games that you can play with the bonus, and a limit on the amount that you can withdraw after meeting the wagering requirements. All of these requirements are in place in order to protect the casino from bonus abuse and fraud, and to force the player to meet almost impossible odds to cashout any winnings.
The online casinos are looking for new players, but they want to bring in quality players that are interested in playing the casino games. It's understandable that not everyone will want to make a deposit and become a regular player, but the casino has put requirements in place that force the player to play the casino games for a certain length of time to reduce the chances that those players are just looking for free money.