With False Prophets the ferengi had already screwed the pooch by installing themselves as gods over a bronze age culture. Also given they came from the AQ-Voyager and janeway felt obligated to interfere. But then again a PD absolutist might disagree.
They're from the AQ, but that doesn't make them her problem. Particularly given that all things considered, they were not doing all that much harm.
But fine, let's say she feels she should stop them. Beam them up, throw them in the brig, and leave. None of this malarkey about saving the native religion that the ferengi have already abused and disrupted, pull them out before they cause more damage and then
leave. Let the natives figure it out for themselves.
Voyager's problem wasn't always getting involved, it was getting involved and then making stupid decisions.
As for counterpoint-Janeway is not without a heart. And so assisting persecuted people is something they would do. Not to mention tense relations with the Devore were pretty much guaranteed given the Devore's attitude towards gaharey.
Janway has an obligation to protect her crew and get them home safely. She does not have an obligation to stop and help every bunch of oppressed randos she stumbles across, particularly not with when doing so puts far more lives in danger.
I would argue she has a strong reason not to do so, since that just invites further involvement and meddling later down the road. After all, you stopped to help one ragteam bunch of refugees, why not stop for the next, and the next, and the next, and now you've taken a 75 year journey and turned it into a 200 year one.
There was that incident where they got into trouble with a telepathic race over B'lanna's violent urges. Which given the cultural dissonance here-wasn't something to be easily avoided.
That wasn't a one-off. It happened in The Chute, Ex Post Facto, Fair Trade, and probably several others I've forgotten.
Janway has to know by, at the very latest, year 2 of this trip that sending people down to trade and meet the locals carries a non-zero risk of something happening. A risk that is not worthwhile, because gaining a little more information about a society that the federation won't ever meet again is not worth the risk of losing valuable personal the ship cannot afford to lose or replace.
And also, the incident you mention was easily avoided. Stay on the ship, negotiate your purchase via the comm, beam it up, and then leave. You can buy spare parts without having people wander around through the markets of a society who's legal system you do not understand.