Because in my opinion this render "pay what ever you want" impossible, anyone
could set a site that looks somewhat official and sell your work.
As long as you agree that non-commercial sharing must not be forbidden, you
can have sites (or P2P networks) where copies of the digital work are
available at no price. That is financially unbeatable. Prohibiting
non-commercial sharing does not change that fact.
This is your conception of ethical?
Enabling or disabling business models does not make a license more or less
ethical. Notice that your arguments would apply to software and you would
argue that licenses that forbid commercial sharing (i.e., non-free licenses)
would be more ethical.
I actually consider that licenses that prohibit non-commercial uses and/or
derivatives are OK for artistic works. Because artistic works are
non-functional (rms' point you have already mentioned), forbidding
non-commercial uses and/or derivatives is not denying the control of the
audience's work. Arguably, the audience does not achieve anything by reading
a novel or listening to a song, or watching a movie, etc. On the contrary,
using software (or reading cooking recipes or manuals or encyclopedias, etc.)
is achieving a work and the user deserves the control on her work, the four
freedoms.
