Comcast uses code injection like this in markets where they implement the 300gb/mo bandwidth cap. Not for injecting ads or anything outwardly malicious, but to

Comcast uses code injection like this in markets where they implement the 300gb/mo bandwidth cap. Not for injecting ads or anything outwardly malicious, but to display a javascript overlay telling you when you're approaching or over your bandwidth limit for the current month. It's a piece of javascript that they'll inject right on whatever page you're browsing (HN, Reddit, whatever).