So yes, it is entirely RAW that touch spells are treated as light weapons.
I disagree. The section you quoted was "You
can treat touch spells as light weapons ..." (emphasis added). This is the same language used for a Rapier:
You can use the Weapon Finesse feat to apply your Dexterity modifier instead of your Strength modifier to attack rolls with a rapier sized for you, even though it isn’t a light weapon for you.
Contrast that with the language used for an Unarmed Strike:
An unarmed strike is always considered a light weapon. Therefore, you can use the Weapon Finesse feat to apply your Dexterity modifier instead of your Strength modifier to attack rolls with an unarmed strike. (emphasis added)
And, of course, you can use Power Attack with a Rapier, despite its special ability to masquerade as a light weapon. The only limitation is you can't use it with 2 hands.
All that being said, the rules for this are pretty poorly laid out. For example, I have no idea why you can Sneak Attack with a spell but a Bard's Inspire Courage is somehow off-limits b/c it uses the words "weapon damage" even though we are talking about what they labeled "weaponlike spells."
My main point is that there is no solid RAW support for saying spells can't Power Attack. And, that the interpretive support for it is thin as well. You're stuck with a sort of "the rules can't say you can ..." type of logic, which isn't very satisfying.
That being said, I don't see that much rides on this. You can't use a "spell" two handed, the only way you'd be able to pull that off would be a channel spell power like the Ordained Champion has, which would render the entire debate moot. And, even setting up a charge with a touch spell would usually take 2 actions, so isn't very efficient. So, at most, the caster is getting +BAB to damage with his spells at the expense of a higher likelihood to hit (going from essentially guaranteed to less than that) and spending a feat. Unless I'm missing something, there's not a huge game balance concern there.