Some of the shows are web only because the studios and TV networks want you, when watching TV, to actually watch the shows on TV or through a cable/sat provider. That's where they make their big dollars (advertising revenue, etc.) and it's going to stay that way. If you're on your PC, that's one thing, but they're likely to block some of the programming for devices that go to your TV like an Xbox, PS3, etc.
This is unlikely to ever change IMO unless the TV networks make specific deals with, say, Microsoft, etc. Other than that, Hulu's available "pay content" is likely to be limited on something like the Xbox or a streaming media device like the Roku.