If you did not read through the terms and conditions of the casino before/after registering on whichever casino that you registered on then I doubt you would have read them even if they showed it to you whenever you followed through with the promo.
It seems to me that this problem could have easily been avoided if you just read the terms and conditions beforehand. And if you did indeed find something wrong/missing then you would be right, from a legal standpoint.
But casinos do not want to have legal problems therefore they create a terms and conditions which covers everything about their website. This also covers promos, obviously.