I don't need to show any "proof" and what you've "heard" is totally irrelevant. There is at least one very large and reputable exchange that has no problem whatsoever with fiat deposits and withdrawal and where you can trade USDT/USD.
you're missing the point.
when you sell usdt for usd on kraken all you're doing is passing the risk of it being backed by thin air to some other chump who chooses to buy it off you believing it's actually worth something.
bitfinex has nothing to do with that transaction and they and the usdt seller are offloading the dollar backing on to some other moron. there is still zero evidence that tether actually has any real money at all.
i can't believe it's been allowed to become as widespread as it has with bitfinex's track record.