You cannot withdraw Tether. You can only trade it for something else on an exchange. Tether does not actually let you convert USDT -> USD. I wonder why.
Presumably you can. I personally do not have accounts at any USDT exchanges, so I can't personally verify that fact. Even if all of the USDT exchange don't accept USD withdraws (which itself is unlikely because nobody would provide sell volume to them), at the very least you can trade them for "real" USD on kraken (at a few percent discount).
You say "presumably you can", but according to David Gerard, there is no evidence that anyone has redeemed Tether. Only people presuming that they could if they wanted to, or people who heard of someone who heard of someone who did it.
The surprising fact is that people do buy and sell on exchanges that have no access to real money.
Trading Tether for USD on Kraken is different, and is the topic of this thread. Given that Tether cannot be redeemed, its price is free to fall as soon as traders on Kraken lose faith -- which, given that Tether has essentially failed an audit, may be happening.
I'm not saying it's impossible. I'm saying I know of 0 reports of this happening - and if it had, I'd have heard, and I mean there's not even bitcoin advocates saying they've done it - and one well-documented one of failing to do so: https://davidgerard.co.uk/blockchain/2017/12/05/bitfinex-rat...