@Gothia

But what you forget is that the Forsaken went on a little killing spree when they got themselves free-will and further reduced the remaining living population of Lordaeron. Death Knights however didn't (in public) start killing innocent civilians.
Also the Death Knights aren't busy 'squatting' (as the alliance might see it) on human (alliance don't seem to thing of the forsaken as being all that human) property i.e. the human kingdom of Lordaeron. Just check some of the quest lines or the wrathgate event, the humans seem pretty much convinced that Lordaeron belongs to them and not the forsaken.
Also since Tirion Fordring played such a big role in the freeing of the Death Knight's it would be too unexpected to think he might have put in a good word for them. After all its not like Death Knight's are welcomed with open arms, I presume it more like the warlocks a acceptable evil.

And those are only off of the top of my head, someone who's really into the lore might even be able to give some decent reasons too.