[SoUlFaThEr]

i think it may have something to do with those ZHLT's Lightflags.....

entities are invisible
to make entities cast a shadow....you can set different settings for light blockage:
normal
Embedded fix
Opaque(blocks light)
Opaque + Embedded fix

it might be these things giving an entity a worldbrush flag so that it will be rendered SOLID......

is that it?.......most of the newer maps have a lot of this on thier func_walls and breakables and illusionaries......but then again it doesnt make sense with the cs_siege thing because the light flags were added LOOOOONG after that map was made unless they updated it for cs_1.5