Many routers still leave telnet open for remote administration, could be the same with port 80, perhaps the router has a web based management console.

I have also seen a linksys web cam that had the telnet port open on it. I contacted linksys about getting a password to mess around with the camera. They denied it existed for a while and then said to leave it alone. I tried brute forcing it for about 60 days (since it was in fact my camera) to no avail. Lost interest after that, but have not noticed excess amounts of bandwidth trying to hit it.