If they prohibit using their services to host publicly accessible servers (however that restriction might be worded) then IMO you need a new ISP; there are all sorts of commonly used services that would fall foul of that and I can't imagine how somebody prohibiting servers in general would stay in business.
They stay in business by offering a reduced rate ($29-$49 per month) to residential customers who do not wish to host a website, but give up the use of port 80/443 for that purpose. If you want to host a website and open port 80/443 to your ip, then you pay the standard rate ($99+ per month).
edit: And some offer the commercial customers more bandwidth during busy hours. They bandwidth throttle residential customers first.