We are talking about mega-corporations that are scared to make anything but a safe call about change. any service competes on three levels. Features, price, and service. Making the wrong call on any of these can take a major company out. Look at how the HP and RIM reaction to the iPad has hurt those companies. Rim was already hurting from its failure to counter the iPhone, and now they are almost irrelevant.
There is a lot of difference between the iPhone and iPad as far as the carriers are concerned. With the iPhones, they cover most of the cost, but get at least $80/mo from the buyers, and it is locked in for two years. Plus the majority are not going to leave those plans. Then you have the iPad. A lot less of them are sold than iPhones, and only about a tenth are sold with 3G/4G. Plus they are non-contract which means there is no sure income for a long period. If they have tethering on the iPad, there is a limited number of devices that can use the tether. The iPod Touch, and laptops. Laptops are big users of data, so they can make a large profit on tethering when peope go over their plan, or buy enough data to be certain. And having the free tethering is an encentive to buy a 3G/4G iPad. On the iPhone, if they have tethering, then the customer is less likely to buy a 3G/4G iPad and would buy the wifi model instead. Data use on the iPhone is relatively limited compared to a larger device, and a person could do a 2-3 Gb plan on the iPhone without issue even with tethering. If they charge $20 for tethering on the iPhone, people are more likely to buy the data plan for their iPad at $30. And the more complex the options, the more likely that customers will just pick a package and stick with it.
A lot of demographics and psychology is used to put these options together. And a lot at risk if someone upsets the apple cart. As a customer, I want more at less cost. However, I have enough of a background in such things as to know some of the headaches involved. Right now, the big issue that few are aware about, is the lack of adequate backbone. The growth rate of data usage is outgrowing the existing infrastructure. We need thousands of miles of new fiber-optic cable laid, hundreds of new server farms, and then we have to deal with individual server bottlenecks. Part of the issue is political and part technological. The government is going to have to create and spend money to develop a semi-secure web that services only government. Probably several layers of security that are not intertwined. For example level one would connect all of the local police and other agencies to data services nationwide, while another level is strictly for military command staff.
The majority of use would need a lot more throughput, and it may require that we move up a technological step in how information is transmitted point to point. Look at a train system. You have one track and you are handling five trains on that track and it is in constant use. You need to handle fifty trains because of demand. You either run ten new tracks, or you find a way to speed up the train traffic on the line until it handles the demand. The cost of new tracks and the time needed to install them is going to limit that. New trains that handle more is a solution, but it takes time to replace all the trains, and there are limits to how much you can squeeze things and how fast the trains go.
. You usually just connect the wifi iPad to the phone via normal wifi or a Bluetooth pairing. No special instructions required for the iPad side. Probably worth doing a quick google search for your phone model to research enabling tethering on it. 
