I think that one reason why Reza could be more neutral (or even more evil - those rats are vicious!) is because it's a primarily -human- town. I know, humans are neutral, but IMHO, all neutral means is that there's no racial expectation to act evil/good. And I can certainly see why Vampires would have more of a hold there, as "the sins of the flesh" have a tight hold on most people. I've always thought of Reza as a border-esque town, with set laws, yes, but not with much respect to those laws. I think the balance of the towns is pretty good the way that it is - Primarily, the towns are all neutral, but you have your good side of town, and the bad. In more ways than one.
Also, to touch on why Plith, the battle-center on the war against evil would allow evil-aligned characters in: Who judges what characters or mobs are evil? Or good? I know one huge part of my Slayer character's background is based off of the fact that she was a murdering, skull-splitting, stab-you-in-the-back pirate for most of her life. But she's still a Slayer, and therefore "good" Appearences, even when covered in scales, and eating kittens, are deceiving.
Anyway, just thought I'd add my two-cents in, as babbling as it may be.