[Schiphol]

I can't be any less of a lawyer, but that sounds OK to me. One is not required to accept the license merely by virtue of it's having been published under that license, but this is surely compatible with the author adding extra conditions to run their program if they so wish?

[hvdijk]

The copyright holder is not bound by the license granted to others, so in general is legally permitted to add any additional restrictions. However, as this particular additional restriction is not one of the additional restrictions that GPL3 section 7 permits, section 10 applies to anyone other than the copyright holder, which states "You may not impose any further restrictions on the exercise of the rights granted or affirmed under this License." This is a problem for e.g. F-Droid's package, which is neither distributed by the author nor distributed in accordance with the terms of the GPL.

[Schiphol]

Thanks for enlightening me :)