Monitoring doesn't need write access. RS-422 is simple and reliable. It uses separate receive and transmit pairs and you don't need to connect both. The freezer can continually transmit its telemetry over unidirectional RS-422 and a separate system can forward it over the Internet.

Hackers can still tamper with the telemetry and create false reports of problems, but actually changing the thermostat settings would require local access, so there's opportunity for somebody to verify the problem really exists.

But why can they control the temperature then?

If I put a Zigbee thermometer in the freezer they can hack it all they want, but I am pretty sure they will not be able to set the temperature to 50C.

If they want to get their pubic-facing remote monitoring jollies, why not implement ro access?

Read access, sure. But being able to turn them off?