...and you pay for it. That's the thing here. It's not like I'm taking advantage of the telcos by abusing a loophole or something. If I go over my monthly allotment, I'm charged X amount plus for every gig I go over.
There is no opinion about that. It's all fact. Tethering costs the telcos nothing. Why? BECAUSE THEY'RE ALREADY CHARGING YOU FOR WHAT YOU'RE TAKING! How does using 40GB on my iPhone cost them less or puts less stress on the infrastructure than using 20GB on my iPhone, and 20GB from my tethered iPad? I'm taking 40GB from them, and paying for it.
Instead, I have to pay an extra $20-30 or so to use a service that used to be built into mobile OSes until the iPhone came out and they decided to monetize the feature.
Nope, you agree to pay according to their terms. That is what a contract is after all.
You assume that a user paying for 5 gigs uses all those 5 gigs every month (at which I would agree with you if that were the case). However, the telecos know that is the minority of people and most never get near that and hence why the prices are at where they are at. If everyone used 5gigs, they (the plans) would cost more than they do currently.
Their argument is that if one normally uses x/5 gigs on their phone, they would then use greater than x/5 gigs if they had the ability to tether as tethering enables the greater use of data use.
As they priced their plans to people using a fraction of the alotment on the whole, this "increase" due to tethering over a lot of people would indeed mean real money to the telcos.
This is not the only industry that prices like this. Meal plans at colleges are prices the same way, as are mileage allowances on leased cars, etc.