I think the underpinning of requiring a species to have warp drive before you contact them/make them aware of other species is that with them having that capability you won't be able to bully them nearly as much as if they didn't, they were stuck on the planet and totally dependent on you for the possibility of space travel and other interactions with all other species/the whole rest of the galaxy, I think that makes sense.
I always figured the "warp" threshold was because hey, they are going to start running into aliens soon anyways, and we'd rather their first exposure NOT be with the Klingons, Cardassians, Ferengi, etc.... no point in hiding and not interfering when they will soon be involving themselves anyways. Its why "Taste of Armagedden" wasn't a PD situation - they were already spacefaring.