Price discrimination. Some people are willing to pay more while others aren't. If you can find a way to charge extra to those that will pay more, or less to those that can't afford it at the higher price, without letting people jump between tariffs to make a mockery of your scheme then you can increase your total profit and better serve your customer base. You can use the same system to more efficiently gouge a captive audience as well though.

http://en.wikipedia.org/wiki/Price_discrimination

I may be wrong, but I'm pretty sure that the networks don't have the capacity to handle as much as they say they do. Add on to that that someone on a laptop is probably going to eat up more data. Multitasking is easier (I generally only look at one page at a time on my phone, but I might have five tabs loading on my laptop at any given time), and there are more applications available that can chew through a connection (MMOs, torrenting, etc).

People use bandwidth differently depending on how they access it. Carriers design their networks and plans around the use patterns associated with particular access methods. Tethering is a difference access method, with different usage patterns, and so may not fit in well with a particular carrier's system.