Are this an accepted standard, or are they confusing to the user (before they check them especially) because they look like radio buttons?
Update: Just found another place that uses them as well.
Update #2: After reading some of the comments, I realized this is present in iOS also, but I have never noticed. Also after going back Morgan's other work, I saw that his original design of this type of check box was a redesign of the current iOS one:
My guess as to where this came from is as a modification of the usual iOS delete button which is round. Since the only time the round check box shows up is when deleting messages, it must have been a clearer way to display that several messages were ready to be deleted, but they didn't give any thought to the fact that is looked like a radio button. It also seems to only be used when removing an item.