As RBUSMC wrote, with the use of the hasty sling as a shooting support, it is what was taught for many, many years. It was well entrenched before the summer of 1983 - that much I know. If one is in a supported position, slung up dealing with targets / threats at distance - the Army qual then was 50M to 300M and there was NO (very little if any) short range work in the general purpose forces - it wasn't counter-intuitive.
It is not a good choice in close and it doesn't mirror how we handle pistols - both of which argue against it as an "A" answer. I also wouldn't completely rule it out if I was across a major terrain feature (valley) and using a sling for support (something that seems to be a lost skill, or at least an unknown one).
Up until I got out of the Guard (now several years ago), its use coupled with bad magazines was the leading cause of base over bolt stoppages.
Regarding the use of the charging handle to get the BCG going going forward - as a left hander, the bolt catch is not my thumb when I finish inserting a new magazine. So automatically going to it is not my default. My work ARs have Redi-Mods on them, so pressing the bolt catch with my short trigger finger isn't a default option either. Pulling the charging handle to the rear and releasing it works regardless.