I mean, the OA rules are obstensibly the same for movement as they were since 2nd Edition's Combat and Tactics first introduced them, and the reasoning I gave was good enough then to describe this very situation.
If you don't understand the fluff behind the mechanics, that's okay. But don't pretend they don't exist... they're one of the few things that -has not- changed.
You might be surprised at what I am capable of understanding.
For example, I understand that the original reason for movement OAs is so that the PCs cannot just rush past the guards and kill the king. The entire reason for the rule at all is to protect the backrank in a turn based sequence game system (typically protecting the PC squishies from monsters getting past front rank PCs). It has zip, zilch, nada to do with lowering one's guard. That is nonsense fluff to describe game mechanic balance and always has been.
Unfortunately with 4E, one can take a wide detour around the guards and still get to the King. So, the OA movement rule still exists but the main reason for its game mechanics existence is less important now. There used to be a shortest path past the guards. Now, that doesn't exist unless the guards cover the entire area. So, the game designers had to add the concept of Fighter marks in order to attempt to offset this deficit. It helps some, but it's still not as strong as the original rules which forced either an OA or extra movement regardless of who was in your way.
So no, the fluff descriptions might work for some people, but they don't make a lot of sense now and never really did. They backhandedly and inadequately explain the existence of rules that really should have been better overhauled in the 4E game system because of how other rules have changed. If the fluff makes plausible sense to you even in scenarios where guard should be lowered but isn't, great for you.
But it's the reason the OP started this thread. They OA rules don't really make sense and never did from a fluff perspective. They used to make sense from a game mechanics balance perspective, but they don't even do that as well anymore.
The original reason for missile weapon OAs (and ranged spells for that matter) is because ranged PCs can hide in cover and be safe and the designers wanted a way to lower that safety in melee. Unfortunately with all of the 4E shifts from powers (and just the normal shift), ranged missile and spell OAs almost never occur in the game system anymore. But, this remnant of a rule is still there, it has just become more or less obsolete by shift and/or shift powers and by most large monsters not having threatening reach (and was a bit obsolete in 3E with the 5 foot step, it's just more obsolete now).
OA rules should have been improved for 4E, but they were not (with the exception of close powers not provoking). To expect that they would be improved to the point that the fluff made sense as well would have been a stretch though. The OA rules are purely a game mechanic adjustment to turn based sequencing and balance issues.
