I think it was a conscious design decision to reduce false alarm rate. Assuming that this low cost device can be/become very inaccurate, then the designers had

I think it was a conscious design decision to reduce false alarm rate. Assuming that this low cost device can be/become very inaccurate, then the designers had two choices: 1) Report all alarms as detected. Eventually, the device becomes a red LED which users learn to ignore. 2) Report a problem when the device is reasonably sure there is a problem. Green light does not necessarily mean that the air quality is good, but when the LED becomes red, it should be taken seriously.