I agree with the supposition that they survived because they just didn't look. Why else would Indy scream for Marion not to look, no matter what? I like the notion that good or evil, regardless of who you are, to look upon the power of the Ark is to die. In that regard, the Ark is fair.
Belloq's summation of the Ark as a transmitter makes some sense. If what we behold when the Ark is opened is indeed the power of God Himself, then not only can it be used as a weapon, but it's too much for mortals to comprehend.
Or, summation:
Belloq, Dietrich and Toht died because, although they were not targets of the Ark's destructive power, they looked at said power, couldn't take it, and their minds and bodies broke. Literally. Like how the voice of God makes your head explode in Dogma.
The soldiers happened to be positioned where an enemy army might be, and they looked, so the Ark automatically fired upon them as shown in the book earlier.
Meanwhile, Indy and Marion didn't look. So the Ark ignored them. Thus, they were spared from both the Ark's destructive powers, as well as from insanity, terror and eventual physical destruction from the sheer magnitude of God's power.