It's not the liability here, it's the fact that cutting corners means you now have more people walking around that your lab said was fine, that aren't, and vice versa. If someone gets a false positive then, following recovery, they'll assume they don't need to take safety precautions like social distancing seriously, which can put them in real danger. And false negatives mean you've now infected the "non-corona" wing of the hospital when your patient with non-corona pneumonia is admitted.