This sort of thing has certainly been the case with the quota-free peering agreements here in Australia in the past. Do Comcast provide for unmetered traffic in their accounting tools? If so it should be relatively easy to test.
The OP really doesn't sound that suspicious really, not really worth the title. If a service provider serves popular content locally via their own client to reduce the hit on their external links what's the problem? Is there some commercial advantage they're gaining from it other than not having that bandwidth eaten up?
If nothing else, it's fairly easy to circumvent for Comcast: Just say that Xfinity isn't an internet service, it's a ComcastNet® service that just happens to get delivered over the same cable as your internet connection. Figleaf: "You can buy a Xfinity/ComcastNet® subscription without an internet connection.".
Where it gets icky is when Hulu suddenly doesn't count against the cap, but Netflix does, and the deal Hulu got isn't available to Netflix.
If a latency-sensitive service decides to host closer to part of its customer base, does that mean they're not respecting net neutrality too? At what point does something stop becoming more convenient for customers and the company (like locally hosted unmetered traffic) and start becoming a neutrality issue?
Maybe I look at it differently being from a country that has always had caps, where the introduction of peering agreements and ISP hosted content meant you got more value (all downloads being 'paid for' to some of that now being 'free'). I suppose if you look at it from the perspective of going from a culture of unlimited data to capped downloads (all downloads being 'free' to most being 'paid for' and some free as it was before).
It's not like they are artificially prioritizing their own content.
Edit: On reading some of the other comments it sounds like it isn't just the same content via different clients, it's actually buying the same content off Comcast? Is this the case? The OP wasn't exactly a wealth of detail.
The OP really doesn't sound that suspicious really, not really worth the title. If a service provider serves popular content locally via their own client to reduce the hit on their external links what's the problem? Is there some commercial advantage they're gaining from it other than not having that bandwidth eaten up?