OP is exactly right. Facebook wants to be a one-way valve, much like Microsoft.
They also don't like the idea of people "polluting" the social graph, either by adding spurious contacts a la MySpace or by removing random nodes from the network.
Well removing is actually making the social graph better as most of the people removed aren't people that should have been closely connected to the user.
One problem is though I guess that there is no set way in which people decide where to cut off there graph. Some people will have friends of friends that they met one and work colleagues just because they work in the same building whereas others may limit to close friends and family.
They also don't like the idea of people "polluting" the social graph, either by adding spurious contacts a la MySpace or by removing random nodes from the network.