they could, but again: then they aren't an internet-of-things device. They could do cool things, but without an internet connection it's not IoT. And the internet connection is the selling feature for most IoT devices.

I can turn my dumb non-internet connected thermostat up if i'm at home, the interesting thing about a Nest is that i can turn up the thermostat before i leave work and the house will be warm when i get home. The internet connection is the value-add. If you don't like internet connected devices that's cool, but IoT devices can't have the same feature set if you remove the internet connection. the call-home is not useless. If you want to argue that certain devices don't need an internet connection, that's fine. and in many cases i'd agree. but arguing that the whole class of devices that is defined by being connected to the internet doesn't need an internet connection is pretty silly.

If you want to turn on the thermostat from work then it does not need to connect to the cloud, also it would be safer not to expose the thermostat directly to the net - have a more secured control server on your local home network and have that one call the thermostat.