For the sake of those jumping to conclusions, there is disabled access at the Phoenix. They have a number of spaces for wheelchair users, and offer free seats for those who need personal assistance.
This is a major tour; some gigs are all standing, many offer a choice. "Manipulative"? "Augmenting ticket sales"? How about simply ensuring that as many people as possible get to see them?
Not everyone going to a folk gig wants to sit down in a comfy chair, and £15 isn't particularly expensive for one of the best live bands around - in any genre.