I think the check box you're referring to is the one when you Create a New Activity in EPP
I agree with you. It should be available on an Edit..... if you look at the source on the page, they are using a couple of functions "loadMilestonePicker" and "disableMilestonePicker" I'm betting it "works per spec" and that they never considered that someone might want a One-Day event to be a Milestone.