This is a common thing on such deposit bonus. You'll find the same on other casinos, did not you read the terms of the bonus before taking it?
I guess so, if you did read it then you wont complain. I have no experience with this site but if in case they have no clear terms about the bonus, you are right to complain.
Always read everything before deciding to take something especially deposit bonus.
Otherwise, you may say the same to all other casinos and it will just embarrass yourself.