They may give you a bonus, let's say in the form of free spins... And then your free spins may end up netting you 20$ in winnings. Well,.you will find out that free spins are limited to up to 5$ per giveaway of free spins... And then you at least have 5$ right? Well, no! You still have to fulfill a wagger limits. And your 5$ had to be staggered at least to a gross turnover of $300! That's 60x... And not only that, you can only wagger on a selection of slot games and nothing else. If the wagering could be done on dice or something with low house edge, whatever. But they really really don't want you to win anything by forcing you to play slots with 5$.
Anyway. When I see something like this for free spins I'll be sure to never use any deposit bonus in such casino. Bonuses don't have to be huge when you're forced to lose everything. Just give a few cents and might as well keep it simple and give nothing. Right?