Some rely on the device itself to enforce it, but that's obviously fragile if you can bring your own device.
Some check the TTL of your packets when they enter the carrier's network, because tethering is at least one more hop and so even if your computer's OS and phone's OS agree on what TTL starts at, the TTL will still be different than expected for your device's platform. Obviously this can still be mitigated by adjusting your TTL, but outside of software that'll handle this for them, that's already beyond a lot of customers.
Some even take the route of only checking HTTP traffic, and detecting tethering based on User-Agent, but I think a lot have abandoned that because it doesn't catch other protocols, and is easily bypassed even on HTTP.
Some rely on the device itself to enforce it, but that's obviously fragile if you can bring your own device.
Some check the TTL of your packets when they enter the carrier's network, because tethering is at least one more hop and so even if your computer's OS and phone's OS agree on what TTL starts at, the TTL will still be different than expected for your device's platform. Obviously this can still be mitigated by adjusting your TTL, but outside of software that'll handle this for them, that's already beyond a lot of customers.
Some even take the route of only checking HTTP traffic, and detecting tethering based on User-Agent, but I think a lot have abandoned that because it doesn't catch other protocols, and is easily bypassed even on HTTP.