For a better understanding, the phrase should be written as "NO-DEPOSIT BONUS" which is a bonus is given to players for free without the requirement of making deposit. In the past, it used to be a free balance upon registration. Just for an example, casino give 5mbtc for new players after registration but this kind of bonus comes with wagering requirement (usually 30x-40x). Means that players can play with this free balance and they can withdraw some (usually there is max withdrawal allowed) after players complete the wagering requirement.
Yes, those bonuses are very restricted and to be honest.
Personally, I do not think there are very worth it, unless someone is looking to try a new casino without having to initially deposit and after the bonus is gone, if the games have proven themselves to be fun enough, then go ahead and deposit a few satoshis. I have always perceived those commercial tactics of giving free balances in exchange of registration are used by casinos which are new or small and are willing to go as far as possible with a limited budget to attract gamblers to their website.
One needs to be careful with casinos with that modality and which we have never gambled before, though, it could be very easy for a scammer to set up a fake casino with free balance and someone convince victims to deposit after they had some "good luck" with the sign up bonuses initially provided by the scammer...I would rather just to stick to deposit bonuses in casinos which have already proven to be legit.
On a legit platform or site, then it would really be just that normal that you would really be facing up those wagering requirement in regards with those promotions and whatever bonuses that they do have
on which it is really just that a typical approach for a certain platform to have. They cant really just give out those bonuses without having those kind of requirement because
they arent really that giving something for free without any requirement or simply having that it would be always have a catch.