How about they don't inspect any content, but I download the media player provided by my ISP and it doesn't contribute to my data cap, but netflix does?
I would think if they have data caps then they will have to determine how much data their own content consumes and count it against the data cap. Otherwise this is abusing their network monopoly to support and provide an unfair advantage to their content business.
In the United States there are often deals where certain services used on certain networks do not count against mobile data caps. For example, DirectTV Now app does not count against an AT&T datacap.