i understand cancelling my account, but if you want to do that you should let me cash out my earned rewards.
Should? The terms of the card say the rewards are forfeited when the card is cancelled. You got those terms with the card.
Sure, some card issuers will be gracious and not hold you to the terms you agreed to. And the cancellations are of course unfair when the real reason is that you've exploited the rewards programs but haven't done anything that is ruled out by the terms. But bottom line, the card issuers have every right to void the rewards--says so in the terms, and there's no law that says they can't follow those terms.