Because the only possible explanation is that they will weaponize it.

If you'd bothered to read TFA, one of the uses they are attempting to create is a DNA 'saftey switch', an easily identifiable string of DNA which marks a DNA sample as sample donated in an investigation and not a forensic sample collected at a crime scene. Such a sample could not be mixed up in a labaratory accident, because no forensic sample could contain that particular DNA sequence.

Of course, they also mention injecting strings of amino acids into bacteria just to see what happens. Still, almost every technology has both good and bad uses.

nobody