Why GPLv3 says additional permissions are removable
As with any copyright licence, software developers who use any version of the GPL can also grant additional permissions to recipients for code that they hold the copyright of. That is, they can say that you can distribute the software under the terms of the GPL, and they can additionally say that, at your option, you can also distribute the software in this way or that way.
About such additional permissions, the following words are proposed for GPLv3, in discussion draft 2: "When you convey a copy of a covered work, you may at your option remove any additional permissions from that copy, or from any part of it." As I see it, these words actually don't change the nature of such additional permissions at all. This topic has come up a few times when I've been discussing GPLv3 with people, so here's my understanding of this issue.
Some people have asked "But if they can be removed, aren't additional permissions meaningless?"