Why the FSF collects copyright assignments.

[freedomfromruin]

The paramount reason Eben Moglen has the FSF accepting contributions 
only with copyright assignment is because the grantor of a license that 
is a gratuity (no consideration (read: usually money) given) can remove 
the permission regarding the use of his property at his pleasure.

(Regardless of the story that was promulgated for the public ("only to 
have standing to sue under copyright, since the GPL is a bare license 
and does not give rise to contract damages") - which was and is a 
half-truth only (Yes: you do need to own the rights to a work to sue 
under copyright, Yes the GPL is a bare license, No: that's not the whole 
reason why one would want the author to no-longer hold the copyright))