Facts:
1. OP is a customer of Xfinity Mobile - which is a "Wi-Fi first" internet service. Note they don't call it "cellular" because it isn't.
2. Their terms of service are clearly laid out here: https://www.xfinity.com/mobile/policies/broadband-disclosure...
To quote:
"Comcast's Xfinity Mobile broadband Internet access service ("Xfinity Mobile service" or "Service") utilizes Wi-Fi service - both Xfinity WiFi and Wi-Fi provided by other Internet Service Providers ("ISPs"). When not connected to Wi-Fi, the Service utilizes our carrier partner's mobile broadband Internet access service network and is subject to its network management practices and controls."
Basically they roam onto cellular when their WiFi is out of range.
3. OP keeps claiming that he never agreed to anything blah blah yes you did. When you sign up for anything with Comcast they make you sign an agreement that you agree to their ToS. See South Park episode "Human CentiPad".
4. The service OP signed up for is at a discounted price because of trade offs. The trade offs are that your device will prefer the Xfinity hotspots over cellular. That's literally how it works. That's why you're getting it for cheaper because the service sucks by design. That's on the customer.
5. People attempting to point these facts out have been getting downvoted which is just sad.