If I purchase a new internet connection, Slingbox, Tivo/and Cable or satellite feed why would I need a router?

Let me clarify. I am based in Japan and this setup in the US at my folks’ house would be solely for the purpose of me accessing the US TV broadcasts from Tokyo. The connection, feed etc. would all be separate from those which exist in their house at the moment. In fact I would not even have a TV connected to these boxes. Just set them up and have them plugged in and sitting in a corner of a spare room. The Sling site and all available sources I have found seem to indicate a router is a requirement. I cannot understand why.

Is it due to a requirement for a computer and the Slingbox to be simultaneously connected to the same ISP via a network during initial setup? Or is it because the assumption is most users will have the Slingbox added to an existing network and in order to allow the internet feed to continue to be used for more traditional purposes, a router would become necessary in order to act a s splitter of sorts?

I am probably missing something very basic, but it appears in the documentation I have read that the only software involved is the SlingPlayer software. What would prohibit me from having it work very well for me by simply plugging into the ISP RJ-45 coming out of the wall?

Thanks in advance,

Mike