Where I live, the operators don't care about it. Even the lowest-end phones support it. As far as I remember, I never owned a phone that didn't support it (the oldest as a modem on serial port and CSD/GPRS speeds).
So what is so special on some smartphones, that it causes problems? As far as I remember, 3G phones (both feature and smart) always supported tethering, whether over USB (CDC ACM, CDC ECM, RNDIS) or Bluetooth (DUN or PAN).
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.
That said, I've managed to tether every Sprint phone I've owned over the last 8 years.
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.
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.