Your ISP could block all traffic except DNS and port 80. They can then redirect your port 80 traffic to their servers using one of several methods such as WCCP,

Your ISP could block all traffic except DNS and port 80. They can then redirect your port 80 traffic to their servers using one of several methods such as WCCP, inline transparent cache,switch with transparent cache redirection, policy routing or a device which NATs your outbound traffic to external IPs to the IP of their internal server.