In my experience in Canada from several years ago, the business teams in wireless telcos don't see it as providing you internet on device x. The ip connection on a device is a gateway towards a service, and the telecoms want to be providers of services, and the differentiator from competitors is having better and different services.
In my opinion the biggest fear of those business development teams is to be treated as or thought of just as providing internet on a device. As being just the pipe is not considered a lucrative market, and open to lots of competition where the race is to the bottom on price. This is also where you get lots of the anti-competitive behaviors, like not investing the effort to make a competitors service optimal within a network, as the carriers would rather launch their own service instead of making someone elses service they don't generate revenue off of work great.
To be fair, in the older gen access technologies, all internet access isn't equal. The access network was optimized for the usage patterns of a mobile browser, in the way it scheduled and idled the radio (I'm glossing over a large number of details). So connecting a computer via tethering that behaves differently and is far more chatty on the network, is actually a much costlier device to support. The current gen techs like LTE do operate much differently, to support chatty devices and apps. With a better optimized network and devices loaded with chatty apps, this difference is probably disappearing.
So if you put yourself in the shoes of a business dev person at a wireless carrier. They'll look at this and say, well we can have a $40 plan for just internet access. Or we can have a $35 plan for just mobile browsing, since that's more efficient and allows us to be more competitive when using the phones browser, and a $15 add on for tethering for those who want it and cost the company more. There are entire teams dedicated to just figuring this stuff out.
And this leads to some of the stuff I would get thrown from time to time, like the $60k bills (no longer allowed in canada, a rule was created to prevent surprise bills, but used to be a thing) when someone bought unlimited mobile browser for $7, and thought that meant unlimited usage for their tethered computer and started torrenting all day which was pay per use.
An unlimited $7 plan only works in the context of the way a user can use a mobile browser, and that's why the carriers think they can dictate how you use the internet connection. They have departments to tailor make plans that only work if they're able to dictate how the internet is used.
I'm not saying I agree or disagree with this model, I'm just trying to explain where I believe this perspective comes from based on my experience. I've been out of the industry several years now, and my opinions are my own.