Also, when you dispute a charge, they are able to put the money in 'escrow', basically, while they investigate... since they control both sides of the transaction (both merchant and customer), they 'keep' the money while they resolve it. If they find in the card user's favor, they deduct it from the merchant account and credit it back to the card user. Otherwise, they release the hold and the merchant can withdraw the money.
It doesn't feel like your money is being held as a card holder, because the 'money' in this case is credit, and it doesn't effect your bank account while it is being resolved. However, it DOES count against your credit limit while they resolve the issue, so it shows you that the money is still 'frozen' while they resolve it. They aren't allowed to charge interest during the dispute, but if you lose the dispute you will have to pay the interest.
This is the same thing that happens when your bank account is defrauded.. the money is frozen, and you can't withdraw it until the dispute is resolved.