That is currently not the case, and I don't think it should be.
If I write a piece of software and release it under the GPL, I can sell it as proprietary code as well, dual-licensed. I am in no way obligated to provide updates and changes under the GPL too.
There are principles of arm's length and equal treatment among equals for contracts to be enforceable. Copyright licences are treated like contracts of adhesion in many jurisdictions (though not in USA). By-attribution share-alike , non-severability and non-revocation clauses in a share-alike licence attempt to make the parties equal, but it does not arrive quite in there, still leaving the copyright owner privileged a bit.
