The poor suckers who have their email server compromised will inevitably be blacklisted by their ISPs. This will effectively prevent any mail from successfully being sent out to recipients. This means that the legitimate email server will no longer be able to send any mail, or any mail of their customers' behalf until the victimized server can be cleaned of malware on their server, and the owner can convince their ISP and anyone else who has cut them off to trust them again enough to accept email from them. It's not good for business when your email cuts out. Lots of money can be lost.
Spammers are assholes getting free advertising at the expense of legitimate businesses and home users. They steal, dupe, and shit on everyone they come in contact with. They obtain their revenue through unscrupulous means. What is hard to understand about the criminal activity here?
"Moooooommmmm!"
"Yes, dear?"
"Bathroom!"
"What's that, poopsiekins?"
"BATHROOM!!!!"
"No job too big; no fee too big!" -- Dr. Peter Venkman, "Ghost-busters"