actually the use of a TV as a 2nd screen is allowed via a PC. use the Chrome browser and start streaming then press the icon (should be in your Chrome tool bar) and select the device and it will start streaming to the Chomecast dongle. (just checked to be sure still worked and it did)
Shouldn't be expected to own a PC computer to stream television service to your television when you are paying for it. If I want to be able to stream it from my phone while my phone is on my home wifi, and my TV is on my home wifi, I should be able to do so. Not everyone owns a computer.
that is the only way to cast to your TV. you pay for TV service. the stream capability is a free service and hooking up a TV screen bypasses Comcast's revenue model.
use a desk/laptop and the chrome browser and you can cast to a chromecast device (it is a grandfathered function).