Many who argue that everything should on the Internet should be free are limiting the medium's potential. They see the Internet as simply a vehicle for expanding the world's knowledge. They assume someone else has already paid for the creation of that knowledge, or that knowledge creation is a hobby. They assume that the broader circulation of a work will benefit the creator by bringing more paying customers to him in some non-Internet environment.
In many cases, such assumptions are correct. But this limited view of the Internet fails to recognize that it can also be used to market products. It may be used to locate a book, get reviews and purchase a hard copy that will be shipped to you. It may be used to buy a pair of shoes, have the physical product sent to you - and returned - if it doesn't fit.
Stock photography falls into the second category. Stock photographers are not posting pictures online as a way of demonstrating their skills in order to get assignments. (Some assignment photographers do, but promotion is not assignment.) Stock photographers have produced a product at their own expense that they hope to sell. Since no one is paying them to produce this product, they must charge for usage.
The Internet is the perfect vehicle for stock photographers to showcase their work to potential customers, but if no one pays for it, the photographer suffers. People have the option of giving away their work for free, but they shouldn't be disparaged for wanting to earn a living taking pictures.