There are certainly some trust issues with Tether, but in this case I don't think that exchanges could easily roll back Omni transactions unless they were to fork the Omni protocol. Omni transactions happen on the Bitcoin blockchain - they are effected by extra data in otherwise normal bitcoin transactions. In order to roll back the Omni transactions you would have to either (1) roll back the underlying bitcoin transactions or (2) convince everyone who runs Omni nodes to update their software to your branch which discards the transactions you wish to roll back.
Option (2) has already happened. Tether got hacked and someone issued/transferred lots of coins to themselves. In response, the whole omni network was upgraded to blacklist these stolen coins!
