They don't offer free tethering because you have to pay for what you consume.
That other companies have the temerity to charge you extra just for the privilege of tethering is a whole other problem. That would be like the water company charging you extra for the privilege of using water to wash with instead of just drinking it.
The fact is, we pay for data plans, unlimited or metered. Either way, it should be ours to do as we wish with! The telcos should not be allowed (should not have any right) to impose on us any kinds of fees or limitations on what we have purchased from them. End of story.
Tethering charges on mobile phones are equivalent to the water company charging you more for attaching a hose to your tap so you can water your garden. Gouging, pure and simple.
To those who say "but people watch videos and stuff on their laptops but just check email on their phones", plenty of people watch video on their phones and many only use their laptops for email. Don't make sweeping assumptions about everyone. The logical outcome to this scenario is charging for data usage by app type. Tethering obscures that, so precludes any move to that sort of business model.
I paid for a set amount of data, not any consumption velocity or data type restrictions. If I wanted to use my month's allowance in less than an hour by streaming some HD video, then that's my choice. The way it works right now, lots of mobile networks are under-provisioned, meaning that even if I want to spend more money on heavy data usage, they aren't in any position to sell it to me.
Don't even get me started on their "unlimited*" (*limited) marketing bullshit. How long can these phone companies and ISPs keep going without providing what their customers actually want?