Earlier quoted context omitted.
> People sending personal emails will be happy to burn a penny of CPU time as "postage" but spammers won't be able to send their spam profitably The problem with this idea is that spammers don't use their own computer, they borrow other people's computers to send the spam. It also heavily penalizes people who legitimately need to send bulk email.
> It also heavily penalizes people who legitimately need to send bulk email. Aka spammers.
I work for an email marketing provider (opinions my own, obviously), and I was extremely surprised how many people genuinely want to receive lots of commercial bulk marketing email. And by "want to" I don't mean "don't mark as spam", I mean "continually sign up for different marketing newsletters, and consistently use links in newsletters/marketing emails to make purchases from multiple companies". It's an alien phenomenon to me, and it may be a minority of bulk email, but it definitely is useful to a lot of people. Maybe it's their equivalent of coupon-clipping-based shopping, or maybe they just like learning about things to buy in that format? I have no idea.
That aside, the last thing an email marketing platform wants to do is send spam. Every time someone marks an email such a provider sends as spam, the reputation of their sending addresses is damaged, and therefore their ability to deliver mail to aforementioned people who want to receive them (and therefore the ability of the marketing platform to make money by facilitating that kind of desired email) is jeopardized.
TL;dr there's bulk marketing email and then there's spam, and surprisingly little overlap (and overlapping incentives) between the people who send each.