Hey everyone, I am the player in question. I am also a mildly experienced GM in my own right. Here are my thoughts to support my argument.
Firstly, I believe feats and feat-like abilities (rage powers, magus arcana, etc.) are inherently meant to bend or outright break the rules. Typically you cannot split a movement up with an attack, but there's a feat for that (Spring Attack). Likewise typically a caster can't cast two spells in a round but there's a feat for that (Quickened Spell). The examples are plentiful here. These feats often come with serious penalties of limitations in their use. Unexpected Strike is no different. Here are it's balancing factors:
1) Class and level restrictions.
2) Must be in a rage.
3) Can only be performed once per rage (effectively this is once per combat without some serious feat or gold expenditure to remove fatigue).
Secondly, as the poster above me has noted, I do not feel this breaks my GM's highlighted text at all. Unexpected Strike adds an additional AoO trigger that the barbarian can capitalize on. Yes, it breaks the AoO rule to an extent - but that is the point. I agree that if I had 15+ feet of reach I should not trigger multiple AoOs for exiting my squares, but I still get 1 for exiting and 1 for entering if I choose to use my Unexpected Strike power.
Thanks for the opinions!