"The GPL, or any other software licence, depends upon the
framework of copyright law. Copyleft is a hack on copyright law
that puts the rights and responsibilities back into the hands of
the user.
"The logic of Richard Stallman and the FSF is that "a copyright
holder in a jointly-authored work is in a weak position to enforce
its copyright unless all co-authors participate in the legal
action". If all the authors participating in an open source project
assign their copyrights to the FSF this puts the FSF in a strong
legal position to act as an umbrella in defence of the GPL and any
code licensed under it.
"Ownership by a trusted foundation or non-profit gives some
assurance of the integrity of the licence, and the FSF copyright
assignment agreement promises to keep the code free; couched in
these terms, copyright assignment is probably a good thing."