Google is the one ad network I'd expect to at least make an effort to scan for and filter malware-infested ads, for one reason: They're a household name and they have other products that actual consumers use.
If there were a headline tomorrow that some random ad network distributed malware onto consumer's computers, everyone would say "never heard of them before" and move on. If the headline were "Google spreads malware onto consumer's computers", it's reasonable to expect that at least a few people would get mad (such as those affected by the malware) and stop using some other Google product(s). So that's a risk that Google has to consider when asking whether malware scanning is a justified expense.
Of course, this doesn't take into account the follow-up question whether malware scanning at the ad-network level (or anywhere, really) offers any sort of protection. (It doesn't appear to be effective, as evidenced by your link.) I, for one, have not whitelisted Google ads (or anything else) in any of my ad/tracking/malware blockers.