It may not be so cut and dry or obvious what is triggering this, but there is definitely some deep packet inspection going on here.
If you notice, going to http://www.google.com/notfound
responds with a real 404 HTTP status code and does not get redirected to the Mediacom assistance page. However, http://login.myinnsite.com/notfound
(also a non-existent page) -does- get redirected. There could be some domain whitelisting going on here, but it appears there's also some sort of basic regex string matching as well.
For instance, I've noticed that placing this on my private website's 404 page disables Mediacom's redirection.
The requested URL <code>/notfound</code> was not found on this server.
<table width=100% cellpadding=0 cellspacing=0><tr><td bgcolor="#3366cc"><img alt="" width=1 height=4></td></tr></table>
This code snippet was taken verbatim from Google's 404 Not Found page. So if any webmaster is frustrated with Mediacom's redirect, simply add that snippet. Surrounding it with HTML comments works as well, as to not interfere with your layout. As the previous poster mentioned, I have also noticed some filtering based on the META generator tag, but it's not quite as reliable. Please note, that redirection never occurs if the filtering software/hardware detects no User-Agent (e.g. Firefox, IE, Chrome, etc).