The GPLv2 is not a contract, it is a revocable license.

Enjoy the read:
http://illinoisjltp.com/journal/wp-content/uploads/2013/10/kumar.pdf

(With full citations).
(PDF attached)

Page 12 starts the relevant discussion.
Page 16 gives the rundown on all the ways the GPL is not a contract.

Later there is a short gloss of state law promissory estopple doctrines, 
but remember: in the case of the linux kernel it, unlike other projects, 
omitted the "or any later version" codicil, and is only under version 2 
of the GPL, which makes no promise of irrevocability by grantor.

(Note: The  SFConservancy conflates clauses that clarify that if a 
licensee's license is automatically revoked for a GPL violation, that 
sub-licensees licenses are not-in-turn automatically revoked)
(Additionally: Clause 0 of GPLv2 specifically defines the "you" in said 
clauses as referring to the licensee (not the grantor), so the 
SFConservancy's conflation is shown to be ever more disengenious)
(Little more that a hope and a prayer to the wind)

So: Not a contract. Is a bare license akin to a property license. And 
there is no "irrevocable by grantor" promise in v2. .: Can be rescinded 
at will.