...We really don't know when the Albino made good his threat. All we have is Dax calling one of the victims a "boy" - but that's how folks address Alexander, too, even when he's a fully mature warrior of eight years.
We also don't know how the UFP would feel about pirates. Most of the "good guy" empires of old (that is, the ones that came on top in the end) were fine with it, and would have rewarded the Albino for what he accomplished. Especially if there was no declared war (although we also don't know if the Klingons believe in such things to begin with).
How Sisko feels about Klingons at the time of the episode is another unknown. He defended the Albino, that is, he opposed murdering him - but he also never suggested trying to bring the Albino to Federation justice, or even to the type of Klingon justice that is not delivered on the spot. The bottom line there being, Sisko made only a half-hearted attempt at best in bringing the Federation viewpoint into this. So he might have been fine with Klingon justice in the end, or disgusted by it, but clearly he wasn't officially obligated to do much about it, which makes him never doing much about it not just a circular argument but actually a point for consistency.
Timo Saloniemi