> "What due diligence and groundwork have you done to prove that they are in fact a terrorist?"

The solution is to make this question their (Clearview) problem. If you are offering a service that makes damaging claims about people, then you need to be liable for any related damages if that claim is later found to be slander/libel.

The risk of a potentially huge ruling/settlement will be handled the same way it is handled in other professions: by paying for liability/malpractice insurance. Eventually, the insurance companies will handle the question of due diligence.