They've been denied the 'right' to use proprietary extensions to the software. Those extensions may provide more benefit to the user than the open source software alone: see also, Mac OS X.
That's no less real a 'right' than the 'right' to have access to the source code, which is something they've never lost because open-source always remains open-source, and they remain free to only use open-source software.
> So yes, the GPL does impose restrictions in specific cases to secure rights in the general case.
Those rights are already secured, because nobody forces users to use proprietary software. If they only want to use software where code is available, they have every right to do so.