As long as there's money to be made
The ISPs are the ones that encourage SPAM. You don't see them raising lawsuits against botherders or SPAM sites. There have been what, one or two successful prosecutions of SPAM operators, and take-downs of only a handful of rogue ISPs that promote the practice. These certainly seem like "window dressing" for the real business of selling accounts and demanding more money to "block" this revenue stream.
Back in 2006 I was working for an airline that - legitimately - wanted to bulk mail opt-in subscribers to a weekly deals newsletter. Yahoo! and AOL both blocked them as a "spammer", UNTIL they started paying a few bucks per thousand emails sent to allow them to reach their recipients.
In Mexico they call this "mordida" - "little deaths" - the bites of the bribes to get your work done.