I know what I'm writing. I know it in a way that I made several objects and collided for the same reasons as techladdie.
If you have 100 FPS on that tile, why spend 5min to modify the object to have 100.5 FPS?
And according to your argumentation, why do we give houses collision properties?
M+R wanted the object to have a collision_mesh, they just wrote it wrong.
Houses are very simple, that's why. I don't think they wrote it wrong, they wrote it really good. In the gaming industry (and 3D in the matter of fact) nobody is using the same meshes for the collisions, because that is way too stupid to do - performance and optimization wise. Yes, you will have 0.5 more fps on that object, but when you optimize your whole map correctly, the numbers are getting bigger and bigger.
Plus lots of times if using the same object for collision, it will have wrong collision properties because it may be too complex, and may even not be able to pass trough.
I'm just saying that that's the right way of doing things, and nobody is going to change that soon. It has been for years, and it will be in the future.