On what basis? Other than ones that commingle USD and USDT and/or are organizationally associated with Tether (which, I think, is mostly Bitfinex and Bitfinex), I'm not seeing where there is much basis for holding the exchange liable for the collapse in value of a traded asset.
But, for example, if it happens to get legally treated as an unregistered security, then the exchange would be fully liable for the losses of their customers, since it was not allowed to sell unregistered securities to general public/unaccredited investors. And this argument by itself seems sufficient to press a serious prolonged case, even if the courts later decide that no, this interpretation isn't the right one.
In general, being an intermediary in shady products may easily mean that you're (also) liable. "Normal" stock exchanges and stock brokers are the exception, they have specially listed immunity exceptions that apply if and only if they fulfil a bunch of conditions; otherwise people may well sue you to cover their losses just because it seems that it's easier to enforce judgments on you than a Hong Kong company.
And they don't have to win the lawsuit, they just need to not get it dismissed outright and scare others into leaving the exchange.
Other than Bitfinex (with he comingling issue mentioned upthread), I understood the main use case for USDT on exchanges is to avoid even touching USD transactions in either direction; for exchanges doing this, they wouldn't be at risk here.
> And they don't have to win the lawsuit, they just need to not get it dismissed outright
Right, but I'm not seeing where you get a colorable cause of action that avoids that for a typical USDT-supporting exchange from a USDT value collapse.
Nope. They'd only be insolvent if they had a legal binding commitment to redeem USDT 1:1 in dollars, and only if they didn't have sufficient other USD reserves to do so.