It seems Romulans at least are perfectly okay with placing their prison planets in gross violation of Neutral Zone treaties.
Where does it say that the planet was in the Romulan Neutral Zone? And even if that had been the case, how would a Romulan prison in the Romulan Neutral Zone equate to a Klingon prison planet in the Romulan Neutral Zone? And again, Nero's arrival point was in Federation space, not in any Neutral Zone, and there is no nearby star at Spock's arrival point. We know from these things that the arrival points are in different locations.