The goal is that people can simply start to play using those characters should they not have time, the desire, or the experience to make their own PC.
Okay, that is right up the alley of what I thought.
As for alignments, having opposing alignments in the pre-gens promotes roleplay ...
Yoda8myhead has a reasonable perspective, however, I haven't seen it play out that way in the groups I have gamed with. If this was a system without alignments, the backgrounds can be interpreted with enough creative interpretation that you might get it to work. However, if you are faithful to the alignment system in the Core Rulebook for Pathfinder I have serious doubts that a group could keep the characters together.
From the LG description in Pathfinder p. 167 "She combines a commitment to oppose evil with the discipline to fight relentlessly."
So if that is the stereotypical take for Lawful Good and they are only 2 steps removed from the alignment, how would Chaotic Good fair being at the completely opposite end of the alignment spectrum from Lawful Evil?
Doppelganger and Watcher both had very interesting comments on the Paladin, which I have yet to read in any thorough detail, although I did notice that they are still required to be Lawful Good.
For me, I found the pre-gen characters to be very interesting character concepts, but I still feel that if you are really playing the characters within their respective alignments, this group will fragment sooner than later. (And that would be especially true for my current set of players -- granted they are unlikely to use the pre-gen characters.)
I guess in the end it comes down to how much emphasis you put into the alignment system versus the character concepts.