Many folks would be happy to allow their photos to be used just for "exposure" or even for fun. Likewise many writers would happily allow their writing to be published for free. This is the case for both aspiring professionals and amateurs who never intend to make significant money with their hobby.

Unions often successfully pressure major players not to deal with these "scabs." There are some good arguements for maintaining a market for professional services by setting a minimum price through collective bargaining.

In a pure market economy, professionals must either demonstrate they are that much better than free or cheap amateur work. Clearly in this case the photographer is much more skilled than a typical amateur and produces stellar work. Unfortunately, in most cases, a publication can easily settle for an inferior free work without any impact on sales.

The marginal cost of a photograph these days is almost zero. Professionals are under a lot of pressure to demonstrate value. I think the good ones can, although many hobbyists produce professional quality work.

In my opinion, a respectable publication should set a minimum price for a work and pay, even of they do not have to. This improves their relationship with their suppliers and encourages better work down the line. Sadly, the economics of publishing may make such generosity infeasible.