I can see why people like Tallius, as much of a whimp he may be, are too honorable to hire the DB or the Morag Tong. But at least some high ranking politicians of the Empire seem totally ok with killing off the own boss, why not send an assassin to behead the Thalmor generals? During the war, the Night Mother was still in Cyrodiil and I'm sure someone in the Imperial Court would have had enogh influence to protect the Listener or at least some top shots of the Black Hand.
The whole great war is pretty much a race war. So why do act both sides so fair in terms of assassinations? In actual history were much cleaner wars than this one and leaders still got backstabbed. Hell, even in relatively peaceful times the Dunmer houses tend to kill each other. Is the DB generally regarded as the bigger evil than the threat of racial cleanings?