Star Trek cast blames the bosses

[Orius]

It is supposed that anyone who can develop warp technology is grown-up enough to deal with on an even keel.

I think it's tied in with the concept of "cultural contamination" and such. A pre-warp civilization, might not be philosophically ready to accept the existance of other intelligent life forms. A species that can develop warp flight however, will eventually (and probably quickly) learn they're not alone in the universe. There's no need to conceal your existance from them at that point.