I think trying to impose that rule would just be a headache. Most common links, by the sound of it, are https compliant anyway but are people really going to double check that their preferred image or file hosting site is https whenever they want to link to something here? Do the mods really want to spend their time dealing with errant posts of this nature (or the resultant queries if such a protocol was enforced via the board code)? I suspect not.
However, BFeely only suggests recommending using https compliance so I see no harm in it being a recommendation.