Image plugin doesn't support nofollow links
While the normal link plugin has support for adding arbitrary key-value pairs to an "a" tag, the picture plugin can add a link, but cannot add a rel="nofollow". The picture plugin is used on the sponsors page with a link for each sponsor, so both clicking on the image and on the separate link brings you to their site.
So currently, it's not possible to 'just manually edit the sponsors page' to make this less attractive for SEO-affine companies.
Unless, of course, the links on the images are removed, but this is a content vs. programming decision.