Because they want to charge you for the expensive "tethering" option. I know, that's fucked up but the US are not very good at forcing consumer-friendliness on

Because they want to charge you for the expensive "tethering" option. I know, that's fucked up but the US are not very good at forcing consumer-friendliness on their companies. On the "how they detect it", unless you're using a VPN they can look at the traffic and easy distinguish "computer traffic" from regular "phone traffic".