Most of the time you will find a free slots bonus at online casinos, while the no deposit casinos normally will exclude other games, especially skill games. The free money is given so you have a chance to check out the game selections and choices but they do have to protect themselves from being taken by mass numbers of savvy players.
Where as slots are a game of chance and it is random whether you will win or lose. Of course if no one ever won at a casino then those casinos would be pretty empty as they do need winners and losers.
The types of free slots bonuses you will be finding at no deposit casinos have free slots spins and this one has been probably one of the best as you can rack up some cash to play on other games or cash out. Free play bonus has a large amount of credits which you are not as restricted on games and you do get a larger amount of money to play with. Every one loves the free credits you can get from the casinos, sometimes requiring a coupon code (which is found on their promotional page) or just instant credits in you casino account. There is more restrictions on the free credits and you really need to read and understand the rules.