But there's no indication of any ships other than the Enterprise or the Valiant (50 years earlier) had ever visited the system. Everyone other than the Federation are apparently smart or respectful enough to acknowledge the Code 7-10 and stay away.
And who's business is it what the two planets do to each other? Neither one is asking for outside assistance. They just want to be left alone. But, like the First Federation and the Melkotian found out, "keep out" doesn't seem to mean anything to Starfleet. Like a 24th century religious zealot, they don't think it could possibly apply to them because they're so good and pure and just so darn friendly so they keep ringing planetary doorbells dispite the "No Trespassing" signs.