I don't think they're trying to hide this fact, even though they don't advertise it.I was a bit surprised that Comcast doesn't explicitly tell customers who rent its cable modems that they're helping Comcast to create an omnipresent WiFi network. I think a case could be made that Comcast customers should be compensated for (or at least not charged for) renting access points that advance Comcast business as well as providing the customers their internet access!
They say the service doesn't lower your cable modem bandwidth, which might be true, since the raw cable has far more bandwidth capacity than anything you could subscribe to.
They also say it doesn't threaten your LAN security, because this guest network provides separate IP addresses and only routes packets to the Internet, which is probably true, although it's possible that a bug in the firmware could be exploited to violate this firewall.
I think you could make the argument that the additional SSID clutters up the spectrum, lowering the signal quality for everybody else - probably true in dense urban environments where spectrum is already pretty cluttered. You could also make a (weaker) argument that the additional SSID consumes more power and therefore raises your electric bills. Maybe, but it would really surprise me if this is significant compared to normal router/Wi-Fi usage without the 'xfinitywifi' network being active.
There was a lawsuit over this in 2014. It appears that the judge suspended the suit, compelling arbitration instead. I don't know the outcome, but I assume Comcast won, because the "xfinitywifi" SSID still exists.
Personally, I think that was correct. People claiming that Comcast doesn't have permission to run an extra network on "their" router seem to forget that when you're leasing the router, it isn't yours - it belongs to Comcast. If you want it to be your own router, you can buy one - Comcast doesn't run this network on purchased routers.