Boost :

terekhov_at_[hidden]

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Notwithstanding the provisions of section 106(3), the owner of a 
particular copy or phonorecord lawfully made under this title, or 
any person authorized by such owner, is entitled, without the 
authority of the copyright owner, to sell or otherwise dispose of 
the possession of that copy or phonorecord.
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Derivative work is a "work". "Copies" are material objects. 
"Works" are the things fixed in a "copy". You can't prepare a 
derivative work without fixing it and creating a "copy". 17 USC 
109 grants you the right to distribute lawfully made "copies" 
owned by you (including copies made by you with the copyright 
owner's authorization [Boost license] or under one or more 
exception(s) in the Copyright Act).
In the Committee Report that accompanied the passage of the 
Copyright Act of 1976 (House Report 94-1976), there is this 
explaination:
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To come within the scope of section 109(a), a copy or
phonorecord must have been "lawfully made under this title,"
though not necessarily with the copyright owner's authorization.
For example, any resale of an illegally "pirated" phonorecord
would be an infringement, but the disposition of a phonorecord
legally made under the compulsory licensing provisions of 
section 115 would not.
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> 
> Anyway, even if it follows from some law text, why is redistribution
> of unmodified copies then allowed explicitely?
It doesn't need to be allowed explicitly (in addition to the
reproduction right grant). You can violate the distribution 
right only with illegal/unlawful/pirated copies (not made by 
you; otherwise, if you make and distribute unauthorized copies, 
it's infringement of both reproduction and distribution rights).
That's how I understand it. IANALBIPOOTN.
regards,
alexander.