Does the Prime Directive only apply to pre-warp civilizations or not?

I’m watching Star Trek: TNG for the first time beginning to end, and there seem to be a lot of instances where Picard is saying that he can’t interfere because of the Prime Directive even though he’s talking about interfering with a civilization that has warp drive. Other times, the crew does stuff with pre-warp civilizations but makes no effort to blend in. Am I missing something about the Prime Directive, or did TNG play fast and loose with the Prime Directive concept?