^ Because he's a Klingon. That's all the reason they need.
And
MacLeod correctly pointed out that Worf killed Duras on a Klingon vessel. Klingons, Worf included, can conduct their own affairs however they feel like - it's sovereign Klingon territory. So by what right did Data and Riker go over there in the first place?
(wonder if Worf could have asked Picard for maybe an hour's worth of shore leave so he could, technically, not be on duty when he killed Duras....and if that would have worked.)
No one in the Klingon empire could openly say anything about Worf killing Duras, because Duras made the mistake of murdering Worf's mate in an inhonorable way and giving Worf every right and duty to kill him.
To claim the lesser Starfleet rules overriding the ancient rite of blood revenge or whatever it's called would have been seen as dishonorable political shenanigans and be counterproductive to saving any claim of legitimacy to power which ultimately was more important to Duras' allies.
Exactly. The right of vengeance is vitally important to the Klingons - so much so that
even discommendated Klingons may claim it (for loved ones, anyway - Duras only accepts Worf's challenge when Worf mentions that K'Ehleyr was his mate).
And would the new Klingon Governement file an objection over that fact that Worf had killed Gowron?
Since the Klingon government doesn't do anything without the Chancellor's say-so...and the Chancellor happens to be Martok, whom Worf put there...I would guess not.