The issue here is the(perhaps intentionally) vague definition of "server". What about the folks in the middle ground? Who want to run a home VPN, who want to use their NAS remotely, or who want to run their own "cloud"? The ISPs could define the term more verbosely and lock out people actually abusing home connections while simultaneously not screwing the people legitimately looking to use their home connection for anything but web browsing.
The more precisely the server provider defines it, the more they are giving folks a playbook for skirting the rules, and as a result potentially encouraging more overall abuse. You're assuming they'll be "screwing people legitimately... " but as long as the customer's total data consumption isn't excessive, an ISP has no reason to come down on them - the ISP should be happy to just collect their fee.
If you want to host services, get static IP blocks, run BGP, pay more for a business-grade service.