Hi ,
This is expected behavior. The foliage is agnostic to whether or not you are spawning on a landscape or static mesh. Because the landscape layer weight only affects the foliage spawning on the landscape itself, it will not stop foliage from spawning on static meshes that are not over a particular layer. However, there are several options for addressing this. One, as you said, would be to add a blocking volume to the areas where you have static meshes. Another, though, is to use the foliage paint tool to erase the unwanted foliage from static meshes affected by them. You can go into the foliage paint tool and determine which meshes you do not want around or on the static meshes in question and erase them without affecting other meshes (such as grass), that you may want around the mesh.