HBO Go isn't an "Xfinity app". The app that's been the cause of controversy is the Xfinity on Xbox app. There are countless blog posts and OpEds about it.
App in question - »xbox.comcast.net/
One of the many complaints - »techcrunch.com/2012/04/15/comcas ··· st-know/
Secondly, since May, the the bandwidth cap hasn't been enforced.
»blog.comcast.com/2012/05/comcast ··· hes.html
It even says it under the meter, "Note: Enforcement of the 250GB data consumption threshold is currently suspended."