They may not have the obligation but then the business is sunk. It would be dust in the wind in days as everyone transfered as much out as possible
It seems they do have a contractual obligation of sorts. The Tether TOS state "The right to have Tether Tokens redeemed or issued is a contractual right personal to you." This language appears beginning in 2016. It is, unsurprisingly, light on the details. But the language does imply that the right exists and is non-transferrable.