Like such could not have been a warrior/mage/warlock/etc before dying and being raised as a DK?
DKs were not race restricted because all that it takes to be one is dying/getting killed and being raised as one.
Anyone that argues for removing class restrictions from races needs to took at what it takes to be the class in genersl, like DK=dead hero raised into undeath by the Lich King, or one of his more powerful followers (Deathlord for instance).