One click toggles the currently checked box. A second click toggles the second box. This is true even when both clicks are on the second box.
I consider it to be a bug, only if that wasn’t the intended behavior.
I’m not sure if this way of using checkboxes is kosher under Apple’s user interface guidelines (which is why I didn’t understand what @billashw was talking about), but I have filed a bug report for this.
Not allowing multiple values is the default. Perhaps that should be changed.