Hi Mark,
I need to be able to hit some pages in my site either using SSL or not and to let that be controlled by how the user accesses the page in the URL
Why?
If the page has no sensitive content why do you care about it redirecting out of SSL?
It does that by design because if a page is not designed to use SSL then users may get browser warnings if the page has some unsecure content, ie an image or javascript with src="http://... will cause a browser warning if the conatasining page url is https://.... So if SSL is available and the page is not marked to use SSL in page settings we redirect out of SSL to avoid the possible browser warning. We use relative urls in the menu so if a user is on a secure page and they clicked a menu link it would result in https on every page they go to by relative links, and again it can lead to unintended browser warnings. Things like the Facebook like button don't work on SSL pages and we can't assume that you have nothing in your content that would cause a browser warning if the page is using SSL.
It is also more work for the server to encrypt pages that don't need it.
Hope that helps,
Joe