The VRChat documentation (https://creators.vrchat.com/worlds/layers/#stickers) suggests those layers to make a surface non-stickerable :

But I noticed today that it is also not possible to drop portals on those layers, the raycast goes through the floor, which could be problematic if we do not want the floor to be stickerable.

I don't think it's a huge issue that's we can't drop portals on some of those layers, I actually think it's nice, but I feel like it should be possible to drop portals on at least some of them.