There is also a loophole defense. There is no policy relating to spying on presidential campaigns, so nothing there to voiolate. IOW, this case is being viewed by Horowitz as no different from some crank call that some unknown dickweed might be a terrorist. Doesn;t take much to open that investigation and not apprise the target. Use that rule against a presidential campaign? No policy says otherwise.
IG's can investigate criminal matters committed by employees within their department and they are supposed to send criminal referrals immediately upon finding any possible criminal activity.
Horowitz sent only one criminal referral.