The Apple-supplied ones are widgets, but they are all displaying a notification of something, whether it's notifying you of the weather, your next meeting, stock quotes, or reminders, all of which are relevant to today. That's why the tab is called "Today".
A tweet, a FB status update, or Philips Hue light buttons are widgets but they don't notify you of anything, much less relevant to today. It opens the door to lots of cool widgets...that have nothing to do with notifications or what's happening today, yet they'll all get dumped there, cluttering it up as a mish-mosh of third-party control buttons, IM text boxes, and other stuff that are unrelated to today's info.
I think you guys are having a hard time understanding that a lot of widgets are not only going to be irrelevant to today, but don't even notify you of anything so why do they go into the Notifications Center under Today?