You are dump? A lot of people use their own computer to making a website like me. And they can open remote desktop too. And some for learn to make website and config network for who want to be technician. And illegal or legal is not relate to ISP. If you open a private server, ISP do nothing to you. The private server only can get sue from official server. You don't even know how the relates between private server and official server. Shut up man. Go google and learn more. If a server have a copyright then they can sue you. If they don't have it, there is no way they can sue you. And ISP just a seller, you pay them, they give you internet. That all. Noob!
Did you ever hear that ISP sue a customer because they making server game?. You dump? Learn more about networking kid. And port 80 just an example, for making a website, they can config to any port they want, just to make them able to config their own WAN IP. My friend got this problem before but he don't know how to config the modem and then he call to his ISP and they helped him. They don't need to ask why he need it because now I can tell 90% of modem can ping their own WAN IP. Just for old modem I think it doesn't supported or may be the firmware not update yet, so it can't pint their own WAN IP