Scopes will often have automatic gain/zero adjust to maximise the amount of detail in an image. In some cases, they will not, or only rarely rarely, do a shutter calibration, as uninterrupted view is more important than temperature accuracy.
For any scope that is handheld, the movement will mean that for nonuniformity correction, it can be assumed that the long-term avarage scene content will be uniform - I think the FlirOne uses this method, and only needs the manual shutter when you want to measure absolute temp.
I suspect if you mount a FlirOne on a tripod looking at a static image, the image will fade out, though they may be using the accelerometer as an input to estimate whether the image should be moving or stationery.
Another aspect is that a scope type device would not need to worry about things like long-term drift with temperature and aging, as absolute values aren't important.