The issue with Tether has always been that they haven't been able to convince the authorities that it is backed 1:1.
As for its usage, it is used in the altcoin market for the most part not bitcoin. So your statement here is wrong on another level too.
Well, what if they buy and it drops even more? At that point they are really supporting at 1 to 1 ratio because they have lost money. Basically courts do care what they do with the money if the end result is not being able to back it 1 to 1. Obviously it is something that always connects to 1 to 1 but how they failed is important too.