Our billing, IMO, is very not up-front about what our limits on tethered/not are. Generally, it hasn't mattered. Whatever our limits are, we're not hitting them.
And it's annoying AF; there is no reason for a carrier to give a crap about tethered/not. Limit my bandwidth, cap the amount I can send, sure — I completely understand not wanting a customer dragging down the network —, but what does it matter what device generated the bits?