Yes you are right that mate and that's the reason why every casino have their rules and regulations or let say terms and condition cause if they give you a bonus then some casino have a task before you can claim it. But some of them that they required specific amount to deposit in order to have or to get that bonuses. So after signing up to or before creating an account then we must research a little bit what is that casino all about if they are trusted or not.
Not in all cases they have such requirements but like deposit bonus most of them have wagering requirements as well withdrawal limits also so nobody abuses these free deposit bonuses and safeguard their interest as well.You should have reviews about casino before signing up and depositing your funds over to know whether it's legit or not.
There's nothing wrong if you get attracted to a casino's bonus and promotion for their new players. But as you've said it's important that you take time reading the reviews of the particular casino to know the experience of the other gamblers especially in the bonuses/promotions offered.
Moreover it would be better to stop relying on these bonuses because certainly it has wagering requirements before you can withdraw your earnings. So if you want to try the casino, just deposit a small amount to experience playing on their platform and see if you'll be satisfied on their services and unique games (if any) that make them stand out to other competitors.