If you find out someone who has used your computer was involved in an act of terrorism, then delete evidence of their guilt, I'm not sure I understand why that

If you find out someone who has used your computer was involved in an act of terrorism, then delete evidence of their guilt, I'm not sure I understand why that would not be a crime. This guy was not prosecuted for deleting his browser history, he was prosecuted for deleting information he knew was relevant to a criminal terrorism investigation.