Yes, you are being scammed. If the exchange is required to withhold taxes (as might be the case under US law if the exchange was operated in the US but the customer is not a US citizen), there is no reason the exchange can't withhold the taxes from the cash payout before releasing the rest. And, this kind of backup withholding is uncommon under US law. I don't know about other countries, but there still should be no reason the exchange can't take the tax withholding from the payout.
