The banks are legally allowed to do that, Tether wasn't. According to the lawsuit at some point they started declining buying back USDT, due to the lack of real dollars in their bank account. That's clearly a fraud, since they were assuring their customers they have sufficient reserves.
Also, the reason they were at some point missing $850M was because that money was seized in various countries as coming from drug cartels. Seems like there's a lot of rope to hang them with.
From what I see, USDT (Tether), is still one of 6 prominent stable coins on Poloniex. Prominent enough to be listed first. If USDT stopped being worth $1 for too long, I’d assume the exchanges would remove it. According to this chart, Tether has done it’s job fine through the ups and downs.
Also, since Tether has its bank accounts in Deltec bank in Bahamas, the inflow of $15B to such a small country would be visible in country's foreign currency balance sheets. The official report from Bahamas government shows that this balance is currently around $60B and has actually decreased during 2020. So this $15B worth of newly issued Tethers are not actually backed by USD (those money never reached Tether bank account), they were printed out of thin air.