It's comforting knowing that your ISP is quietly installing software (probably written by the lowest bidder) onto your computer, isn't it?
Anyway, this is a problem that's been around for a while, although it's been slowly rolled out in different areas at different times, starting around 2006, IIRC. The fix is simple (but annoying), you have to call up Comcast and run through the gauntlet of helldesk folks who will give you the runaround, until you reach someone willing to help, often a manager/supervisor, who will ask for a few bits of information, then provision (make an exception to) your connection, allowing you to use Linux. The problem is, they LIKE having their software on your computer, for various reasons that are obvious enough.
Sometimes it's a piece of cake and you can get the connection opened right away, other times it's like a vacation at Gitmo...but be persistent.