Bittrex does not have a USD-USDT pair available for regular traders. They are literally just testing this service this week with a limited pool of users.
Here’s what they are said in what is the most recent post on their news blog, May 31, 2018:
https://support.bittrex.com/hc/en-us/articles/360004397871-B...
>During this first phase, fiat trading will be limited to approved corporate customers in the United States located in Washington State, California, New York and Montana. Qualified international customers may participate in the market as well. Like all customers who want to be active on Bittrex, fiat customers will be required to submit to both the standard registration process --including proof that they are operating in qualified U.S. or international regions -- as well as other specific fiat terms and conditions.
You are trippin’
As to who might have a vested interest in maintaining the appearance of the dollar-tether peg, come on. That’s not too hard to figure out.