The 100+ free spins no deposit bonuses that casinos offer are not completely free in the true sense of the term. Every online casino is a business finally, and profit is a driving factor behind each of them. Therefore it is in their interests to ensure they do not run into losses by offering free bonuses.
The 100+ free spins are available for free. You do not have to make a deposit to claim them. Effectively, this means you do not spend your money while using them. However, you will need to make a minimum qualifying deposit into your casino account to be able to claim winnings from them.
Also, every online casino offering this bonus puts a cap on the maximum winnings you can claim out of the total amount you win with this bonus. This clause is also inserted in the terms and conditions associated with the bonus to ensure the casino is not left high and dry by offering this bonus. Imagine picking up a $5000 win from a no deposit bonus, and this being the case with 10 players in a month! The maximum cap on wins from the 100+ free spins no deposit bonus is usually put at between $50 and $100.