I imagine it's because the cell providers asked for it.
The first 2 or 3 google phones I had let me turn on tethering with no problems despite it not being in my contract and somewhere around the pixel 2 it stopped working. So it seems like an intentional change on their part, and likely one that Apple also enforces. To change the flags mentioned previously you also need root, so it provides quite a speed bump for folks trying to use tethering despite it not being in their contracts.
The first 2 or 3 google phones I had let me turn on tethering with no problems despite it not being in my contract and somewhere around the pixel 2 it stopped working. So it seems like an intentional change on their part, and likely one that Apple also enforces. To change the flags mentioned previously you also need root, so it provides quite a speed bump for folks trying to use tethering despite it not being in their contracts.