Bethesda is just the publisher. The developer is Arkane and none of their prior games have required always online for singleplayer, even their prior game which had an emphasis on online elements. The reason it's even surprising at all is coming from Arkane's heritage.
Arkane has been (hostile-)acquired by Bethesda a long time ago and the founder Raphaël Colantonio has since left to form a new studio. It's better ot think of acquired studios as brands than separate entities, even if they still keep their own physical location open.
Ultimately, this decision rests on Microsoft and considering that they also acquired Activision-Blizzard recently it's not too surprising. Heck, even Windows has been blurring the line between an offline install and an online service.
