How could you differentiate between a spell like protection from good that detects as evil from the character it is cast on?
That would be entirely up to the DM and how he wants to describe the scene.
So do you think they specified evil elementals but not evil undead by mistake? or clerics of an evil deity but not evil clerics?
I really do understand your point. But bottom line, IMHO, is that detect evil does just that: detects evil. Not good or neutral.