Re: Just why?
Part of it is content licensing, and part is to do with broadcast area laws.
The registration says which region you are in, which they use to restrict which channels you get. Content licensed for NSW can't be broadcast to WA, for example. And commercial stations can't broadcast outside of their area either.
I dont know if they actually enforce this - my old Aurora (the predecessor to VAST) card let me access every Aurora channel. And I'm not eligible for VAST (not that I was eligible for Aurora either - I got the card from a remote school I worked for at the time who were chucking stuff out).