Does iOS 6 or any other iOS OS have drivers or kernel modules for an HP printer?
Source of everything I'm about to say: I'm an iOS developer.
No.
Your iOS device does not contain the necessary drivers to print via USB - and here's why:
Developers
We are kind of, well, lazy. We don't want to write more code than we have to - we want to write one thing once and have it work.
Now imagine all the iOS devs now have to take into account that you can print from USB. Now we might have to write code to take that into account, there might be subtle differences in the drivers, etc.
The nice thing about AirPrint is it just works. Not just from the user's standpoint, but also from the developer's standpoint. Developers can basically 'layout' the paper just like they would a bunch of views onscreen, send it to a print controller, and let it handle everything else.
Nice and easy.
You
You're a user. You want things to just work, right? Even as a developer, I want the stuff I'm using to just work (See Stack Overflow for thousands of examples).
If Apple put in kernel drivers and USB printer drivers and all whatnot, the user now has to manage that, and Apple has to conform to HP's, Canon's and Brother's requirements.
With AirPrint, the manufacturer conforms to Apple - a much better experience for you and me, the users.