Under GFDL if a person decide to sell contents of CZ, he is allowed to do this. GFDL permits commercial use. Per example, if a big corporation wants to make a printed encyclopedia with all contents of CZ and sell it for a expensive price, it can do that without any restriction. Many people would work for free while others would profit from their work.
If CZ restricts commercial use, nobody would profit from the job of others. All CZ material would be free for everyone. No one would be allowed to sell it.
As I ever said:
Why should be allowed commercial use? That idea is good for software, but why for an encyclopedia? Only a few people in the world would be able to buy a wonderful printed encyclopedia. Why not the same rights for everyone? And why some people have to work for free and others earn money with this?
I don't see why that's such a big problem. The big win for CZ will be to get a reputation as a provider of excellent content. It's a positive thing if others pick up and help to distribute it. The text will remain free---such a book could be put under the photocopier or scanned into computer, and any of the editorial additions or changes made could be fed back to the project.
In any case as has already been noted, non-commercial licenses restrict CZ's own commercial possibilities. It might be very worthwhile to collaborate with publishers to produce commercial books from CZ content, with royalties to support CZ.
