Let's suppose I buy 100 USDT on an exchange.
What Tether claims happens:
0. Bob and I own $100. Tether and Bittrex own nothing.
1. Bob sends $100 to Tether Limited.
2. Tether limited sends Bob 100 USDT. They now have $100 in their bank account.
3. Bob transfers 100 USDT to bittrex.
4. I send $100 to bittrex. I buy 100 USDT using that $100 from Bob.
5. Bob withdraws his $100 from Bittrex. I withdraw my 100 USDT from Bittrex.
6. Now, I own 100 USDT, Bob owns $100, Bittrex owns nothing.
7. Tether Limited has to hold $100 in reserve. It's not their money - it's supposed to be backing USDT. In addition to having assets of $100, they also have $100 of liabilities.
The reality:
Steps 0-6 are the same.
Step 7: Tether Limited buys a vacation to the Bahamas with their $100, and ignores the $100 liability. They are actually insolvent, and I have no recourse against them, because the Tether TOS make it very clear that I cannot redeem my USDT for USD from them.
The whole point of tether is that it is supposed to be backed 1:1 with USD in Tether Limited's accounts. They have not proven to anyone that this is actually the case. For all we know, all that money went to the Bahamas, or a Swiss bank account, and they are just issuing USDT, pocketing what people pay them for it, and keeping none of it in reserve.
It's really not hard to believe when you consider how many millions of people are on these exchanges. Binance touts 3 million users, and has 425,819,642 tethers. 141$ average a user is completely believable.