They can. However, by the 3.5 rules, Detect Evil detects "Undead" not "Evil undead".
http://www.d20srd.org/srd/spells/detectEvil.htm
Similarly, it detects "Cleric of evil deity" not "Evil cleric" (LE deities can have LN clerics, CE deities can have CN clerics).
Whatever energy keeps undead...