My understanding is that a license is not a contract, but simply permission from one party to do something. A license can be part of a contract e.g., when a license is provided in exchange for money. However, if I just plonk an open source license on a bit of software I wrote, there's no contract with anybody.

It's unlikely that invalidating an open source license would be of benefit to a user, since they then have no legal right to make copies of the software.