When I was checking out agency average salary statistics last year - in my market QA salaries were slightly higher than developers. From memory it was only in the junior and intermediate categories, but still.
I think a lot of this comes from many "QA" roles being more than just your basic testing role - eg banging on a GUI randomly and recording the bugs. They are often domain experts who know how the system should work, and every detail about what it actually does. So not only do they test for random software bugs (eg. "The program crashes if I click things in such and such order..."), but also fine grained functionality details (eg. "The results of this calculation are wrong, and it should be rounded off to cents correctly, not just dropping the third decimal place.").
Finding good "domain expert QA" people is hard. And companies that work in certain niches want to hold onto these people like grim death, once they find them. So I think this is where a lot of the QA salary data gets pulled upwards.
I've worked in roles where it would definitely be harder to find a good domain expert QA person than a developer. The developers in those cases didn't really have to be domain experts at all, we were just fed the formulas, screen designs, and told to get crackin'. Much easier to replace, at least on a junior/intermediate level, than a good domain QA person who knows the industry and the business inside out.