Part of the value of the web is that it need not be confined to browsers. As an open protocol, content can be parsed by a variety of clients, and/or wrapped up in candy-coated shells (Fluid, etc), and/or served up via API for the benefit of native apps and mashups. Twitter wants to have their cake and eat it too: to benefit from the open web while contributing back as little as possible.

Also, the Twitter website is currently a terrible poster child for web apps: it's slow and bloated, to the point that I'll sometimes forget what I was going to tweet by the time it fully loads. If we were talking about the old site, which was clean and minimal HTML with a handful of progressive enhancements, it might be a different story.