We have a problem getting people to accept free licenses. One of the issues is that, at least in some countries, conditions stating that the authors allows any usage of his image for any duration are ruled illegal by courts (should the author complain), as abusive clauses. The idea is to protect authors from abuse by greedy publishers. I've even heard some lawyer from a photographers' society explaining to me that free licenses (including Creative Commons etc.) where thus illegal.
Lately, in a private email, Greg Maxwell made a remark that I had made in other circumstances, that is
For example, surely the 'share alike' nature of copyleft contracts prevents the contracts from being considered unconscionable. Such licenses would be acceptable for our purposes.
Indeed, that's one point that we may argue: with "sharealike" licenses, the authors do not cede all rights with no compensation or insurance; they rather have strong guarantees that uses of their work will not stray from their intent of distribution of free content.
That's one argument one may want to pursue when contacting sources.
Regards, DM