Eh, this is kind of right but also not really responsive to the thread.
For one thing, if the author provides the source with a GPL license to user A, and user A sends it to user B, user B has the software under a GPL'd license. The normal reason for dual licensing GPL/proprietary is so that user B can pay money to bundle the software in a non-GPL-compliant way. The author can stop licensing future releases under GPL, but they can't revoke the GPL on already-distributed software.
For another, this isn't what's happening here. GNU Parallel is released under the GPL, and the author is affixing what is debatably an additional term to the GPL'd release, under the claim that it doesn't count as an additional restriction because it's "academic tradition". By the same token, I can add a clause to my software saying that rich people can't use it, because it's "hippie tradition" to stick it to The Man.
For one thing, if the author provides the source with a GPL license to user A, and user A sends it to user B, user B has the software under a GPL'd license. The normal reason for dual licensing GPL/proprietary is so that user B can pay money to bundle the software in a non-GPL-compliant way. The author can stop licensing future releases under GPL, but they can't revoke the GPL on already-distributed software.
For another, this isn't what's happening here. GNU Parallel is released under the GPL, and the author is affixing what is debatably an additional term to the GPL'd release, under the claim that it doesn't count as an additional restriction because it's "academic tradition". By the same token, I can add a clause to my software saying that rich people can't use it, because it's "hippie tradition" to stick it to The Man.