How does that work for free email newsletters? They're typically already paying an email service provider. Having to pay even more could be onerous for large, double opt-in lists, even though they're not spamming.
I'm guessing that 99% of newsletters are sending vastly fewer messages than the people who send me half a dozen "Welcome to Adulte Sex " messages a day.
Maybe, but spammers are also more likely to be sending from rented botnets that can handle the increase in CPU usage spread over a large number of other people's computers.