"Because it is no different surfing the net with the iphone displaying the data or your laptop displaying the data."
I'm not sure I fully agree with that statement. Your behaviour is likely to differ between iPhone usage and what you do on your laptop. e.g. on your laptop you're more likely to exchange large documents via e-mail, download Windows updates, etc.
There will be some concept of what an "average" user does and their current tariffs will be priced accordingly. Changing the device being used is likely to change the assumptions.
Controversial suggestion: maybe what you want is a capped usage tariff that can be shared between iPhone and tethered devices, rather the current "all you can eat"....