"It looks like the HTML mode is intentionally missing from the Global Announcements tab due to administrators using them in such a way that it causes problems in their settings tabs of their accounts. The guides in fact show the Global Announcements without HTML editor mode." (Jesse Poulos -- Canvas Tech Support)
Our use case: When creating a Global Announcement that has a link to an Instructure domain (like the Canvas Guides), those links open up in the same tab/window...essentially taking you out of the Canvas interface. Links created in Global Announcements that go to other sites like Google, Yahoo, CNN, etc. open up in a new tab/window automatically. As a work-around, for those links with Instructure domains, I've had to use a URL shortening service (ex: Bit.ly) to make a short URL for my links so I can "trick" Canvas into thinking it's not an Instructure domain. This is an extra step that could be eliminated by having all links (no matter what their domain) open in a new tab/window. Or, at least could we have the option to choose whether or not we want any link to open in a new tab/window or not?