Brion-
In fact I find it downright insulting that I'm not trusted to mark a spelling correction as "minor" without logging in.
On which basis should you be trusted? When you're not logged in, nobody knows that it is you. I've seen quite a few vandals who liked to test Wikipedia's defenses by claiming to make spelling fixes while in reality vandalizing the article. Marking edits as minor has one primary purpose: Being able to filter them from RC. And that should only be possible if there is a basis for trust.
In addition to the trust factor, there's also the advantage of a simpler user interface for first time users.
Regards,
Erik