This means that proprietary derivatives of GPL-licenced sofwtare could conceivably be made under Fair Use as long as they satisfy one or more of the criteria used to evaluate Fair Use claims. This is true of GPL-2 licenced work as well, and the FSF’s GPL FAQ make this explicit. Nobody has exploited this as far as I know. It may not be possible to claim Fair Use of source code in a way that is harmful to Free Software.
But if it is possible to make Fair Use of GPL licensed source code to create proprietary software or to distribute binaries without source this results in a number of ideological and practical double-binds that will need some subtlety and a sense of irony to resolve. Even if it is not actually possible this will not stop Fair Use claims against the GPL being funded by the usual suspects. It is important that the Free Software community considers Fair Use before this happens so that a measured and effective response can be made where appropriate.