It is rough. There are many ways that different tools put NAs, na, N/A, "", etc. in a file. To chose only "NA" would mean I would be excluding the output of other tools. I chose accessibility over specificity. #trade-offs.
Why? "NA" stands for "Not Applicable", but a blank cell in a CSV could represent any number of things of which "Not Applicable" is only one.
Perhaps it would make sense to have a "pretty" mode and a "literal" mode (which would also turn off the clever processing of numbers)?