Why Dark knight alignment?

[Fortheseen]

So... no dark heroes allowed?
Thats a pity.

I would see this in many shades of gray. Another good example of this would be warcraft III with Arthas. He was willing to burn down towns and kill people who he knew were already plagued by that disease just to save what was left of his kingdom. He thought he was doing the right thing but was he really? Another example, it's kind like the whole thing with drow. Most other civilizations view them as evil since they've grown up that way in the under dark but not all of them choose to go that route. Sorry to get sidetracked there but I personally don't think there should be alignment restriction also it depends on your DM.  If you were going to go with a 'good' dark knight I would suggest changing some of the alignment base abilities more towards his/her actual alignment and make sure the background you have for your character sounds convincing for your DM but then again maybe s/he won't mind it.