I think "innocent" and "guileless" also bracket the sense you're going for, but they don't quite fit either.

Like, he doesn't see the malice in other people, but its not because he's innocent/naive of such intents, nor does he lack the skills to look for it (guileless), but because (as you say) he doesn't care if people take advantage of him, up to a limit.

Properly calibrated, that's really admirable.