You bring up a valid argument but, unfortunately, it cuts both ways.
Aircraft manufacturers are liable when their vehicles crash due to malfunction.
Aircraft manufacturers are liable when their vehicles crash due to malfunction.
If this happened anywhere that policy and law said it was okay to use autopilot then you would see changes in the policy, and fault laid on air traffic control at minimum, and then the scope in which autopilot could be used would be massively restricted.
If boeing had advertised the feature for use at low altitude in congested airspace and written the training with that in mind then most likely someone at boeing would be criminally at fault.
Similarly, if some airplane instrument is stated to require regular calibration to stay within acceptable error limits, I don't think the manufacturer would be liable if the instrument starts to drift when those calibrations are not carried out. Or if some crash-contributing decision is made based on assuming a higher accuracy than promised.