I'm not sure I agree with this. It sure didn't like Belloq and the Nazis, but it spared Indy and Marian, apparently because they showed it the proper respect.
Respect, that's what it's about. No wonder the Ark was so mad. "I get no respect, I tell ya. An archaeologist dug me up once. Then he started talking about Noah! No respect!"
Actually, what I got from that scene was: If Marion and Indy looked into the eyes of those wraiths, they would have 'melted' as well.
The only thing that saved them...(as lampooned in Robot Chicken) was keeping their eyes closed.