My guess is that an unlimited plan on your phone doesn't actually pull a lot of data if you just use it as a phone. It's a different story after you tether it to your laptop and start downloading entire movies or throwing a 10GB directory into dropbox. Maybe I'm missing something, but that's my guess.

Most unlimited data plans I've seen have some sort of throttling clause, basically stating that if you use more than X GB of bandwidth in a month they reserve the right throttle you to Y Kb/s for the remainder of that month.

I think actually that's just T-Mobile. They definitely seem to be the most progressive of the US carriers.

Most carriers either cut off your data entirely for the rest of the month, threaten to terminate your contract, or charge you an outrageous overage fee.

The "problem" is that carriers would rather seel $40-60/mo datacard plans. Tethering cannibalizes that.

That said, I've managed to tether every Sprint phone I've owned over the last 8 years.

They sell differently priced plans for datacards and phones? That would make sense.

Here they sell just data plans. Whether you use dedicated modem or have the plan on your voice SIM, it is your problem. Some operators allow to share the data quota among multiple SIMs, so you can have both. Because the price is same, so there is nothing to cannibalize.

Did you pay a flat fee data rate per month?

Currently yes, I pay flat fee per month, however, my plan is speed-limited.

For the high-speed plans, there is always stated data limit. If you want to transfer more bits, like movies or sync big folder mentioned above, it is cheaper to use fixed connection (unlimited at commonly achievable 40 Mbps FTTH is about the same price as 20 GB over theoretical 14.4 Mpbs HSDPA; about 24 EUR/mo, both by the same provider [Orange]).

But still, if you want just few GBs per month, do not need fixed connection, why would be the provider bothered by tethering? Browsing and mailing is still more comfortable with laptop, than phone.