The vanity "value" to an amateur may be enough to just stroke his ego. A price is established by what a willing buyer and willing seller agree too. Unless there's duress, it's not theft or tantamount to theft. A photographer just has to say "no". If it's a one-of-a-kind photo, than it would be worth monetary value to the buyer. But if the buyer can get a similar photo for the cost of "vanity", than that's all the photo is worth, despite what you think it's worth.
This may sound cruel, but that's how it works. Better the photographer understand the market and adjust his game plan to create a photography niche that he could sell to and make money. Tilting windmills or shoveling s**t against the tide is no way for a businessman to operate.
I think it's unquestionably true that to survive as a photographer you need to have a product that is more than a typical amateur can consistently achieve. There's a lot of ways to do this: you can have a unique vision, unusual access to subjects or locations, consistently reproduce great results on demand — something, anything to be competitive.
Pope seems to have this. He has images that cannot easily be reproduced and he's made these kind of images consistently over time. Still, the band has decided that however the market has valued amateur photos, should also define the value of his clearly professional images.
What makes Pope's argument so powerful is that he's not just complaining that someone asked for a freebee — but that this band organized their entire book project on the assumption that they would need a $0 budget for photography, that they're not paying for photography even though they clearly need it and need it to be good. It's only icing on this cake that the band has been so outspoken about musicians, who face similar market forces, getting screwed by people expecting free material.
One more point —
Just because the market creates a value or lack of value in something does not absolve people from ethical responsibilities when it comes to playing a part in that market. The band understands this when it comes to music — if we want good music, we need to pay good musicians. I have no doubt that the market would respond favorably to child labour, as it does in other parts of the world, and it would value it cheaply (as it does in other parts of the world). You would find desperate sellers and buyers willing to take advantage of this desperation. To simply say it's 'cruel, but that's how it works' is to relinquish personable responsibility and ignore the power we have to act collectively. Another option is to lobby for education and change. It's not always tilting at windmills — a fact proven by the lack of a child labour market in the US despite obvious utility.