Because they are custom objects, and MA maps are not allowed any custom objects from my understanding.
It's an attempt to keep the map size down so when people need to download maps they aren't waiting 30 minutes, as well as a control over framerates (i.e. making sure the custom object(s) doesn't kill people's computers).
I do wish they'd open it up a little though, as Broadband is becoming more and more prevalent. But that's just a wish. =)
