Yes, on the T-Mobile "One" plan they did throttle tethering traffic. You had to pay an extra $15/mo or so for the "One Plus" plan to get LTE tethering speeds. I believe they may have lifted this throttling on the new "Magenta" plans they rolled out a few months ago.
> But increasingly, those upgrades come at the expense of sellers, who often build their businesses on Amazon and have few other places to generate revenue.
That's a huge risk that's been known from the beginning, though. I feel for these sellers, but building a business that's 100% dependent on a 3rd party (especially one like Amazon that you're also in competition with to some extent) seems like a poor situation to place yourself in.