The examples you cite, Anonymous (get a Gmail account for your avatar, fro goodness sake!), are not so "simple" as you want them to be. What you are describing as RL "derivative works" are based upon the use of what are legally "public images".

SL is not a "public" place, in fact. The environment is, in reality and legality, owned by Linden Labs. It follows, therefore, that the content created therein is NOT public and is NOT simply "appropriated" in the way you suggest. Why do you think that the LL spelled out specifically that user-created content is owned by the creators?

If Strawberry agreed that her work could be "photographed" (I hate the use of that word in this context) and re-used, that is HER decision and RIGHT.

Do your homework, and do it outside of SL; you're getting some very bad information if you believe that "Artists have been challenging these laws for years." Go here, http://creativecommons.org/ " and READ what it says about YOUR work, not someone else's work. What rights they may have given are only your concern if you wish to incorporate or re-use THEIR work. Protect YOUR work with the license.