Tethering on a plan that doesn't allow it is like showing up at an all-you-can-eat buffet and leaving with a backpack full of food.
This is why that, when phones tether, those tethered packets are routed separately so that the cell carrier can throttle them when needed to maintal quality of service for everyone else.
By tethering/tunneling through your normal connection, they can't do this, and if this became an epidemic they would either need to do thorough DPI and heuristics to detect and block the tethering/ban the user, or over-provision their towers to handle the varied traffic volumes of both regular cell phone activity and people watching 4k Netflix on their TV through their phone.
launches bittorrent client on the phone
You can get plans that support tethering or mobile LANs - they aren’t even that expensive. Carriers will usually prioritize those connections lower than public safety or mobile phone connections to ensure better user experience. LTE and 5G fixed home plans are an easy example of this available to consumers.
"You are paying for a water service, what does it matter how the water is consumed?" Though of course there are big differences. An obvious one is that water companies aren't profiting off these restrictions like mobile operators at least partially are. And since water is either heavily regulated by or entirely ran by governments, the cost to the consumer doesn't necessarily represent the true cost.
(The California restriction even seems reasonably well enforced. When buying a 2.5 GPM shower head on Amazon[2] you'll get an error if you try to ship it to a California address. Most eBay sellers enforce this as well, though not quite all of them.)
[0] https://www.federalregister.gov/d/2020-27280/p-56
[1] https://www.build.com/ca-compliant/c133273#:~:text=Residenti...
[2] https://smile.amazon.com/showerhead-2.5GPM-that-wont-ship-to...
What's egregious is ISPs also enforcing _how_ I'm allowed to consume the data I'm paying for. So the fork analogy is much more appropriate.
See also: modern intellectual "property" laws & enforcement, the businesses that push for it (typically large companies with a wealth of IP), the organizations that facilitate the control of information (governments, Microsoft, Google, Apple, Netflix, etc.), and the business models that depend on it.
There are cases where the benefits are worth a reduction in freedom, but this ain't it.
A better analogy is a gas station which charges more for gas that goes into sports cars than gas that goes into minivans.
Premium gas costs more. Race cars need race gas — that costs even more.
Where ISPs cross the line is by trying to also enforce _how_ I can consume the data I'm paying for. Having plans that restrict tethering is consumer-hostile, plain and simple. Whether I'm tethering or not has no relation to how much data I consume. They can continue to restrict bandwidth and data limits if I go overboard, but I'll be damned if I allow them to tell me how I can use it.
So if we're going with the buffet analogy, then it's like them saying I can only use a fork to eat, as someone mentioned above.