Although the answer already given is an explanation enough, here's some additional reasoning against changing the guard alignments.

One, someone mentioned that there are few places for evils to rank, so let them have some city guards. Let them get wanted flags for it too, because this leads into the second reason not to change it.

Two, evil city guards would promote the good people killing them, which could not only get them wanted flags, but isn't that good no matter how evil the guard. Killing someone who is loyally defending their home town isn't a very good thing, is it?

Three, the good alignment of the guards tends to keep good people from killing them, which for lawbreaking good PCs leads to an interesting RP conflict. If you're good and get attacked by good guards, you should run to make sure you don't kill them. They're guards, they care more about defending the city than about morals, so to them if you've broken the laws, you're evil and deserve to die. Being a storm giant or an elf isn't a defense, if you're fighting a guard you must be a bad person.

The reasoning here may be why the area designers chose good aligned guards, and it may be why future cities might also have them. Then again, the reasoning I put forth may have nothing to do with those choices. Either way, unless you can come up with a strong reasoning other than 'fitting the feel' of a city, the guards will most likely remain good and orderly people who live to defend their homes.

---Quislet, answering this for anyone who would actually care, other than Graatch.