That's where I also get confused.
So far the reasons I could come up with are:
- the exchange got bought out silently and is now owned/controlled by the people behind tether
- the exchange got paid to use tether (for example 0.5% of the volume as incentive)
- the exchange holds a part of tether's fiat as guarantee
- the exchange is run by people who are inexperienced in appraising counterparty risk
- the exchange is run by people who like to take risks
- the exchange expects to cover any potential losses with the profits they made so far
- the exchange has been offered proof that the 1:1 peg is genuine (backed by the Chinese govt using US bonds)