For a little more thorough of an answer, the reason is that Power Attack specifies that off-hand attacks only gain a 1:1 increase. Double Slice does not modify the wording of power attack, or the wording of how the off-hand weapon is treated.
If Double Slice were to do what you wished, it would have to state that the off-hand weapon is treated as if it were a main-hand weapon for the purposes of feats, abilities and such. It could even have made a specific mention of how it interacts with Power Attack.
As is, Double Slice simply modifies how much of the Strength modifier is applied to damage with an odd-hand weapon.
It is, however, not an unreasonable houserule that you might consider discussing with your GM.
For what it matters, according to Hero Lab, Double Slice, does, in fact, add full damage in conjunction with Power Attack, at least with a double sword.
I don't know if they have any special input from Paizo, or whether it's just their (HeroLabs) interpretation (or just a glitch), but it certainly seems well within the intention of Double Slice and certainly makes the book keeping easier.
Sorry, one of our players has a character with the name Soltan, wondered if there was a connection. It never seemed that common of a name.
Interesting that herolab applies the full bonus.
And I do understand what' you're saying about the double slice feat not specifying it modifies power attack. Conceptually it seems to me to make sense because of how TWF fighting and off hand attack works but without a dev clarification or a house rule you're probably right.
No, I don't LARP.
I meant just that. The feat makes no mention of modifying any other specific rules aside from how much of the strength modifier is applied to off-hand weapons. It would have to modify a rule that Power Attack depends on (such as modifying the type of attack it is considered) or directly modifying the wording of Power Attack.