I could be wrong, but I wouldn't really think tethering to a tablet would be using more bandwidth than the phone. You're just redirecting your data signal from the phone to the tablet, it's not like you're watching videos on your phone and tablet at the same time. Yeah a pc might use more bandwidth for the same video stream, but who's using their mobile phone data to connect their home internet? If they are, that is a problem. I don't think it should be an extra charge to tether to a tablet, or a laptop, while you're out and away from wifi. Once you use up the data you have paid for, then you get slowed down or charged more or whatever the plan calls for. If their systems detect you are using massive bandwidth nonstop, all day long, as if you are using your MOBILE data to connect your home internet setup, that's evidence of abuse and you should get cut off. Other than that tethering should just be a given with no charge.
Maybe a device limit I can see. Like if you're tethering 5 devices at once than you could just be giving your friends free internet when they should be paying for their own.