The games I have seen this happen do give you borders around planets you own, but they dont enforce a colonization block to other empires.
Like in GalCiv, each race starts in a star system with their homeworld, and a second, very useless planet (Which with terraformation gets pretty good), but due to how low quality it is, you focus on grabbing the good planets around, and sometimes the AI, not finding any good enough planet, moves towards this second planet, making a colony on your face.
In GalCiv it's not an issue since culture flipping a planet is a thing, and a fledging colony cant compete with a homeworld on that, so I actually always let the AI colonize it, so I get a free planet.
I hope culture flipping or something like that is in Stellaris, it's a cool way to gain planets without war, and sort of helps enforce more concise borders, since any planet in the middle of other empire is likely to flip.
EDIT: Oh, new comment as I posted.
Yeah, that seems like shared systems, but planets? That's rather more complex imo.
I wonder about those borders though, not sure how to interpret them, like, which empire has the stronger hold on the system?
The borders seem to me like they are like GalCiv's in that they show culture spread too.
Notice there's a system just behind the 'O' of the Themlar Union that is JUST by the edge of the red empire's border but lacks the overlap near it, meaning it's only colonized by the Kingdom, yet it cannot exert culture pressure that well, so the border is just by it.
Whereas the borders between the Kingdom and the League are clean. (Though they too have a bit of overlap, despite no planets anywhere near...)