They give away the free bonuses to bring in more quality players. While the no deposit bonuses will often attract bonus abusers, they also attract many new players that will eventually make a deposit. Also, these bonuses have terms and conditions to protect the casino from abuse.
The free bonuses are an easy way to bring in new players. The more extravagant the bonus, the more players that will flock into the casino to take advantage of the offer. In most cases, the new players will leave without winning anything on the no deposit bonus because of the wagering requirements, or they will enjoy the gaming experience to the extent that they will reach into their pocket and make a deposit.
Free bonuses are a marketing ploy that has worked well for the online casinos over the years, and its one method that they've been able to use to get the land-based players to try the casino games over the Internet. Most of the land-based casinos offer rewards for a certain amount of play, but very few of the land-based casinos will give out free cash to anyone that walks into the casino without any past gaming experience.