A defense against license revocation is the existence of a contractual relationship between the copyright-owner and the licensee.
However, where no such relationship exists, no such protection is apparent.
Obeying a preexisting legal duty (such as to not commit copyright infringement by using/modifying/etc a work without permission) is insufficient to create a contract.
Illusory promises are not binding upon the grantor.
For those who have chosen to not pay the Grantor of a "G" "P" "L" (GPL) license, the license can be revocated at the will of the copyright owner.
""retroactively""
Remeber: non-exclusive licenses do not transfer any rights. Only permissions (license), which can be revoked, and without a contractual agreement such revocations do not give rise to damages against the Copyright owner.
Nothing gets you nothing.
>WE'RE GOING TO DISBAR YOU, YOU'RE NOT GOING TO BE LICENSED FOR LONG.
Go fuck yourself, enemy.