By default, you have no right to be on or use private property without permission. MIT's network is private property. It might be "open" in the sense that MIT liberally gives out permission to use it, but that doesn't mean they give up the right to revoke your permission. This is the same as my lawn. Just because I let people cut through it doesn't mean I give up the right to single you out and keep you from walking on it.

The example you give is an edge case, but I think technically the "get off my lawn" revokes your right to cut across it as other people do. I don't think this is a pretty common sense definition.