Hulu is trying to keep its content restricted to traditional browsers. By doing so it hopes to shut down clients such as Boxee and other apps that enable the websites content to be distributed over non traditional browsers.
This appears to be an on going battle against non traditional means of displaying Hulu's content. The reasoning may be revenue based as the site may not receive proper views or clicks when viewed in such a manner. More than likely it's probably also upsetting major corporate networks who don't want to see Hulu streamed to televisions. Either way, when there is something to be hacked or cracked there is a team somewhere dedicated to the cause.