I've always wondered about this too. What exactly is the reason they scrub leads in the first place? Do they have some kind of filtering system in place that can tell which leads are no good? I've even noticed sometimes when I'm testing out some leads with a good proxy and a legit email addy, I won't get credited for the lead. These are the same offers I'll notice getting really bad conversions later on when I start to promote them. It just depends on the offer/advertiser. This makes it really hard to test offers, because it's hard to tell if the visitors are just not putting in their email, or if the offer is just not converting no matter how many people enter an email.
So is just a problem with email/zip submits? If so, has it always been like that?