The rule says "A creature can take a free action to use an attack power only once per turn."
But you're right that it was to limit the number of MBA in a turn.
Solution: Change Dual Weapon Attack to "No Action". Voilà.
It does.
I have to disagree with this. If indeed "Power Strike" qualifies for "an attack power" then what you're saying is that in a single standard action you've used TWO "attack powers". The first was your MBA where you rolled that d20 to hit, and the second was "Power Strike". That's like saying my Vanguard Sword does 1d10 extra damage on a charge and counts as a free action attack which is clearly not the intent OR how it actually works.