My guess is that compliance with accessibility guidelines may well save them a lot of money. The case nearest to this I can think of was of a folk music organization that squandered a huge proportion of its funds on a revamp of its visual imagery after being conned into it by a graphic design firm.

The result was they got only minimal improvement in their image, had the accessibility of their information fucked up to an extent they still haven't recovered from, they lost sponsorship funding from institutions that undertandably didn't like seeing their dosh thrown down the toilet, and they came close to having the whole organization go bust. And that was before the economic crash.

Has Sing Out! published their accounts? What did they pay for that shitty design job? Do they realize how much cheaper a good one would have been?