Whatever the case, you are strangely attempting to justify carrier pricing schemas through customer costs alone, as if the carriers were driven by sheer necessity rather than the desire to exploit their leverage in a deficiently competitive market. Maybe that's not what you're suggesting. If it is, my breaking even point still stands. If it isn't, and you agree that the pricing schemas are driven by leverage exploitation, then my larger point that carriers will abuse their power to the detriment of the public still stands.
I'll grant you one thing, which is that your "usage models" argument is sufficient to explain why tethering restrictions exist at all. But it's neither necessary nor sufficient to explain the specific pricing schemes, which I chalk up to leverage exploitation. And don't try to tell me carriers would never exploit their customers--they have an established and unambiguous record of doing exactly that.
I'll be leaving this conversation now, since I'm pretty sure you're in this to attack me rather than have a genuine discussion. Take care!