Then should they prevent me from buying a Mac Mini/Apple TV, hooking it up to my TV, opening up a web browser with Hulu and watching something in full screen?

Then why don't they shut Hulu down?

1. Hulu is where they're testing the waters of Internet distribution; it's still a limited experiment.

2. They're hoping Hulu -because of its requirement of a computer- won't cannibalize their broadcast TV shows. In terms of their bottom line, Hulu > pirated shows on the Internet, but Hulu < TV.

Edit: nikblack explains it more eloquently in this comment: http://news.ycombinator.com/item?id=486620