Politics. Pragmatic considerations. The Cardassians could make good allies, etc. As noted before, DS9 was a deconstruction of the Trek philosophies. The Federation was just being practical.
the Klingons were stationed at DS9 just before they went into Cardassian space, so Sisko could have argued that they'd already dragged (albeit indirectly) Starfleet into the situation.
Like anything else in fiction, the PD is probably just another plot-device used to advance the story rather than being an actual rule. --Sran
As stated above, one good reason for the Fed to intervene: The Klingons were docked at DS9 before they left to invade Cardassia, which could have put the Fed in an awkward position.
Some reasons they didn't have to intervene: Cardassia was involved in so many schemes to violate the treaty that you have to wonder if it was worth it.
One moral question for the Fed is why they would sign a treaty or ally themselves with a power that practiced genocide and mass oppression in the first place.
Unlike with Bajor, the Fed refused to sit by when it came to the Cardassia.
In one episode they provided a 12 industrial replicators with military support. Kira said Bajor only got 2?
It seems unfair that a power that brutalized millions of people, gets priority treatment while others who are more peaceful and willing get secondary treatment.