Indeed to get the free spins, it is not required to deposit but in order to wager the winning from the free spins, deposit is required.
Are you talking about Sherbet.com no-deposit free spins? You can complete the wagering requirement by playing with the free spin winnings, they aren't asking for deposits in that case. But you have to make a deposit of $20 at least in order to make the free spin winnings withdrawable. The deposit can be made after completing the 45× wager.
But still this promotion cannot be considered as a 'no deposit bonus'.
There is no way that you can withdraw your money made from free spin by first depositing the money at sherbet, so we should not call it a "No Deposit Bonus". If you do not want to withdraw your winnings, you may not deposit but if you have won and need to withdraw, the deposit is required.
I think the term No Deposit is cleverly used by Sherbet in order to attract more gamblers as people get excited that they can get bonuses without depositing. I am sure there must be many new signups seeing this promo of the No Deposit bonus.