Time Warner Cable has been doing this since back when they were Time Warner Cable. I had this popup in like 2009. I yelled at the guy on customer support and he was like "dude we bumped your internet speeds to the point where your modem can't hang" and I was like "there's plenty less shady ways to let me know" and he was all "dude I just answer the phones, dawg" and I was all "right-o, you still work for a shitty company" and he was all "truth, peace out." 2009.
Comcast? The shitty thing about Comcast is you can't hotspot if you use anything other than Comcast's DNS servers. And Comcast's DNS servers redirect you from Google to Comcast's shitty landing page. But of course Hacker News is all twitterpated about fucking modem detection.