Cheap as in if FLIR paid more for the lens it could be better. Material costs are less relevant as you can make a very good lens just from ZnSe, or pure Ge, or silicon or even Ge-chalcogenide if prepared to pay for added processes, lower yields, extra elements or other complications of doing a good job.
The best lenses can often include several materials to optimise the design
Or you can do 'good enough to sell' for low end models and hold back the better performance (& higher costs) for the next model up which you charge for (in) appropriately.
I have just run the E4/E8 lens data, and it is interesting given the images you posted above.
45° FoV on a 320 x 240 @17µm is a 6.6mm focal length
FLIR state f/1.5
The near point of the camera is stated as 0.5m
That combination says best focus at 0.7m with a far point at 1.2m
Or to keep infinity in good focus you set for 1.7m, and the near point is 0.85m.
These are the problems inflicted by fixed focus, or 'focus free' as the spin doctors in the FLIR lying department have called it.