Copyright law reserves certain acts, including modification and redistribution, to the copyright holder. The Debian project only has permission to redistribute under terms as granted explicitly by the copyright holder in a license grant. Thus, if a work is received by the Debian project under "GPL plus extra terms", then, if those extra terms place additional restrictions beyond the GPL, Debian cannot satisfy both those terms and the terms of the GPL section 6; and thus, under section 7, may not redistribute the work at all.
You do not have addressed my points. Why does Debian can not satisfy GPL + extra terms? The GPL + extra terms is the conditions that Debian must satisfy. The GPL forbids _Debian_ to put additional restrictions; it does not says that Debian cannot distribute a GPL + additional restriction software if these extra conditions has been put by the copyright holder; since it is not Debian who have put these additional restrictions (and the copyright holder cannot violate the GPL).
Of course if these extar terms make the software non free then it is another problem.
Olive