A prepaid cellular modem is installed to transmit essential telemetry.

The idea of TVs doing this smells of urban legend to me, but a friend bought a new car recently, and he can connect to it from his phone (obviously over the cloud), he was wondering how it works because he didn't have to subscribe to anyhing, but yeah it has cellular connectivity paid for by the car manufacturer...

It's required for use in vehicles, at least in the US. Tracking cars and even remotely disabling them is important for safety reasons. But it is suspicious.

This would be a great thing to hack to get free internets. =D

Those SIMs usually have very limited bandwidth packages.

Great, make sure you max out that bandwidth with random garbage (preferably to the same endpoints that the telemetry goes to) to prevent your owned devices from exfiltrating your life.