Per user limits are not easy to implement, maybe easier to visualize - consider that there are a bunch of packet gateways sitting behind a load balancer and each HTTP session may end up on a different server. There is no entity that counts the live bandwidth usage on a per user basis, let alone control it. Billing and metering is done on a session basis through logs. So from T-Mo's point of view it is much easier to detect a HTTP session as video and just throttle that session.
It is very easy to implement this (I have worked on such a limiter before). You pick the point of entry into your network (wi-fi connection, ISP connection), keep packet and byte counts for every such point of entry, and limit them.
ISPs have it especially easy, because they can be assured of being able to distinguish traffic from a given user (hardware control of the medium). It's a bit harder in wireless scenarios, since the client can spoof multiple different IDs, but it's hard for them to keep open a TCP connection under those conditions.
Just because your cheap home router does it doesn't mean it scales to thousands of users on one router. Some home routers are actually quite capable AND very unsaturated. I'm not trying to defend carriers but it is a very apples to oranges comparison.
We did that on low-end PC hardware 15 years ago for conference and guest networks (800+ simultaneous active users, LAN and WiFi).
I find it unlikely that Linux, FreeBSD have gotten less efficient since then and the hardware has made enormous improvements, far in advance of the common uplink speed.
