What Is Comcast/Xfinity WiFi Code Injection Doing?

This sounds like a Front Porch appliance - 'PorchLight/7.0.1406.154 Server'

http://www.frontporch.com/

I built a similar system several years ago, and modified the html directly using kernel modules and a non-blocking proxy. These JS based systems like front porch are pretty easy to build, and often give crappy performance since they are using blocking proxies.

So what is it doing?