But Gmail is a webapp, not a website. You don't get a link to an email stored in Gmail. You can only access things once you're already inside.

Twitter is very different. If you're signing in yourself to update your own feed, that's one thing. But when you are trying to view the feed of someone else, that's a horse of a different color.

The question really is: should Twitter be an "app" or a "site"? If you allow links in from the rest of the web, it should be a "site".