Oh, and if "their ways" involve a death penalty for accidentally falling into a flower bed, then they deserve to be told exactly how wrong they are.
So where exactly do you stop? Do you tell the Klingons that they can no longer settle legal matters with ritual combat to the death? Do you no longer allow Vulcans to decide ownership of a mate by ritual combat?
Or do you only force your ways on those who can't protect themselves?
Who exactly is the Federation to tell people how their societies should work? IDIC? As long as we agree...
EDIT: Just as an aside, the cultures that Kirk changed were usually by product of him freeing the Enterprise from an assault by the same computer that was controlling the society. In The Apple, he was actually trying to leave Gamma Trianguli VI and was blocked by Vaal. Picard chucked the Prime Directive to save a single person, one who should've never been sent to the surface in the first place. Regardless of what Yar's report said.