Comcast injects JavaScript into webpages to show copyright notices to customers
The root of this problem is that there is no "control channel" (or whatever you want to call it) from an ISP to its customers. Email doesn't work because ISPs don't always have the customer's address and email can get spam filtered. Paper mail is expensive and may not be read. Until someone defines and implements a protocol for this, ISPs are going to keep inventing weird kludges. I wonder if Hotspot 2.0 can be adapted for wired networks.