All I know is that I have always loaded max and never had a problem that resulted from having them fully loaded. My understanding is that the original spring and follower from when the 30round magazines started being issued were sub standard and thus the short loading was a stopgap solution. Since the late 80s there has been an improved spring and follower and more recently there has been a 3rd upgrade to the follower.
As far as SOPs go, that just tells us the guy writing it is older, or at least that is my guess. I have seen smoe units with some pretty fucked up idiot stuff in their SOPs, even units in hot AOs. Then of course I have also seen units that didn't have an SOP as far as anyone knew, LOL.
While I was in Bosnia I was with both American and Swedish engineering teams on occasions where they were using82A1s for destroying anti-tank mines and in both instances the teams were fully loading magazines, maybe they were just unaware of the flaw? I dunno.