The specification is section 3 of the GPL
And reading the license closer, the restriction seems to be intentional. The last paragraph of section 3 provides that if the distribution mechanism is access to copy from a designated place, then making source code available in the same way counts as distributing it, even though third parties are not compelled to copy the source.
In other words if you put your executable on a website and the code along with it, that is OK. But if you ship someone a CD, then you have to be willing to ship them sourcecode as well.
(Don't ask me why.)
Cheers,
Ben
