Because I don't feel that an author should be allowed to restrict the distribution of his or her works, at all. It's exactly the concept of Death of the Author [1], just on a slightly different level.

If the author forbade, say, the translation of his work into French, or forbade the reading of it by people with brown hair, or forbade the reading of it on browsers other than Internet Explorer, I'm sure you'd agree with me that such a restriction is insane, and would refuse to cooperate with it.

This restriction he is trying to impose is a bit less crazy, but he has just as little right to make it.

[1] http://tvtropes.org/pmwiki/pmwiki.php/Main/DeathOfTheAuthor