The "snapping" is because the POI designers set the density so it does that. This has positives and negatives. The positive is that it looks nice outside. The negative is that the terrain goes inside as well in order for that to happen and POI designers have to adjust the builds to make it so you don't see that. And there are vanilla POI where you can see it.
I agree that it would be nice to have the ability to set density with a tool in the game itself even with the negatives.
For now, if it bothers me, I just place a sheet block around the edge, laying flat on the ground, and then paint it so it looks decent. Or I'll place hedges around the edges.