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.
Free spins are free to experience and enjoy games while can figure out your strategies in a game you like. It helps you to avoid losses with your trials and can increase your probability to win with your deposit money.
Asking to have free spins and allowance to withdraw what you win from free spins are just too much in my opinion. Free spins actually are attractive because people will not have fear that they will lose money by playing games. They will be more ready to create accounts, explore Sherbet and play games. Chance to convert them from free spin bet to bet with deposit money is real.