There is a limited amount of throughput a tower can handle, both on its backhaul and antennas. Depending on what you're doing over that tethered connection, you might be using up 2x-10x the throughput they provisioned for "you"; if enough people tether at once during rush hour, there's going to be a significant drop in speeds for both you and people just trying to use their phones normally.
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.
In general a few dozen or even thousand techies on HN doing this across the US isn't going to change anything, but it's obvious that we'd eventually have a huge problem if everyone moving into a new apartment or house decided to forego wired internet and instead stream exclusively over a hidden tether to their phone. This is why, when actual fully-supported home internet over 5g is available in an area[0,1], availability tends to be limited and people still sometimes get deprioritized.
If your use affects everyone else in the area, it matters. It's like having a bonfire in California in fire season; sure, it's your property, and your firewood, but you can't pretend it can't possibly affect someone else's enjoyment of theirs.
Because you’re not buying that. The use cases used to build the solution have to make assumptions. Microsoft Outlook uses exponentially more network resources to fetch and send email than a purpose designed mobile app, for example.
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.
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.