This is all marketing. All the speed tiers, all the add-ons, etc. are all marketing. There's no technical reason for any of it. 5G has huge bandwidth, and the fiber backhaul has huge bandwidth. The most that can be argued is that a saturated link needs fair queueing applied to it to give customers fair bandwidth while the company installs more capacity.

But you can't underprovision and then charge a premium for a fast lane if you are running the company on technical principles.

If you asked for justification, the carrier would say that tethering is usually used for business needs, and the increased use would be unfair to non-business users.