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.