Google's acceptance standards are minimal, or at least they have been in the past. And once publishers are in the network, they can use their AdSense code on any site (even if it violates AdSense's program policies) until they get caught.
You are assuming that Google would use the same criteria for accepting publishers to AdSense in their sites.
To make this work right, I think Google should provide some programatic interface so publishers could automatically feed their current newsletters issues before mailing so they could insert their ad tags. If a publisher is not approved for placing ads in newsletters, the Google ads could just show a blank space.
It's possible, of course, that Google would limit the e-mail use of AdWords to major corporate partners, but that wouldn't prevent "AdWords spam: It would simply limit such spam to high-volume mailers.
If Google would accept placing ads only in opt-in newsletters that would leave professional spammers out.