I have only heard of CoinPoker from the three casinos whose welcome bonuses were mentioned in the OP. I can picture their red logo in my head.
Generally speaking, welcome bonuses aren't there to give away free money. If that was the case, casinos would be out of business quickly. The rules aren't in your favor and it's made so that the big majority of new players will lose the bonus funds given to them. The casinos don't lose much if some players manage to meet the wagering requirements and players are under the impression that they are filling their pockets with free money.