An E-mail relay is not even in the same ball park as http traffic through port 80, although it might surprise you to learn that a lot of people do use personal E-mail servers for their home and businesses. I know I have in the past. I have a good ISP.
As for why I use port 80, that's what it is intended to be used for. Most home server devices are configured to use port 80. In the future, all homes will be connected in some way. Even appliances.
Just out of curisiosity, I checked out 10 trusted Comcast IPs and nearly half responded on port 80. Most of the responses were from routers. Test it yourself. The results may surprise you. Open port 80 has nothing at all to do with proxies. Ban them if you want, but you will be blocking a lot of visitors who aren't using a proxy. Far many than those who are using a proxy. In fact, I can't quite understand why somebody would think that port 80 was somehow synonymous with proxies. They're not.