Just thought I'd quote the OP again as I think the key is in the first 2 sentences.
"Just wondering what the point of Rule 14-2b is, when caddies can line up their players and then move away just before they start their swing? Why is it not assistance when players are lining up, but becomes assistance when they make their swings?"
That for me is the issue. What is the restriction in Rule 14-2b on making a stroke while your caddy or PP is standing behind you trying to stop?
It doesn't doesn't stop a caddy or partner lining up the player as that happens before the stroke. So if the intent is to stop that, then it seems to it would be better if the Rule did say that this shouldn't happen once a player has taken his stance.
Decision 14-2/1 Player Aligns Partner's Club Before Stroke confirms that the Rule doesn't apply prior to making a stroke so clearly there is some reason why the Rules guys think that makes all the difference but I can't really see that.
Would be interested in shedding some light on what the intent of the Rule is? If it is to prevent caddies or partners lining a player up, it seems to me it doesn't really achieve that.
I suppose it's to stop caddies coaching their players, and probably why some amateur tournaments have a ban on professional golfers acting as caddies. Nevertheless helping with alignment is still coaching in my eyes. The rule should be changed so that assistance starts when the player takes his stance.