Having struggled with it, here's the most charitable reason I can give:
First, remember that the blind have always been able to order pizza by just calling in with a phone. Domino's is serving the blind in that sense.
The case here is about them being unwilling to provide an online-only discount to the blind. So why are they paying so much to defend, specifically, a blind-inaccessible, online-only discount?
They offer an online discount, I think, because they get so much more valuable data from it. Like most websites, sadly, their site loads you down with a torrent of trackers.
What does that have to do with being accessible? Well, if the blind can easily navigate to the important parts of a site, so can their screen-reader ... and so can any program. Then, anyone, not just the blind, can bypass all the crap on their site when ordering, and even shut out the trackers.
Where's the flaw in my logic?
tl;dr: Anything that helps the blind navigate their site also helps you bypass tracking, and makes an online-only discount unprofitable.