...But perhaps only because it's so damn expensive to send assassins all the way into the heart of the UFP?
Seriously speaking, when K'Mpec says "your life in the Federation would not be affected by this judgement", he's apparently simply saying that Worf will be a sentenced criminal but the sentence will not involve any elements that would significantly inconvenience Worf's everyday affairs. It's not as if Worf would have been put to death inside the Klingon Empire, either, had he done what K'Mpec is speaking of and accepted his father's smearing. He'd be the son of a dishonorable traitor, then, and forced to compensate for that if he ever wanted to be somebody himself (the way Klag compensated by denying his father in "A Matter of Honor"), but none of this would affect his UFP Starfleet career.
Mogh was chosen as the "traitor" since the High Council did not expect Worf to be "Klingon" enough to pose a challenge. They essentially thought the House of Mogh was defunct since they presumed Kurn was the son of Lorgh.
My understanding of Worf accepting discommendation is that it was the best compromise available. K'mpec had no civil war to deal with, Duras wouldn't be dishonoured and his family would be in good standing, and Worf would be out of the way and could still resume his Starfleet career.