If this story is true, how could Mattis let this happen?
Well, Sec. Navy was responsible for issuing the letters, so I would guess he would have the final say. According to the Thomas More Law article, it alleges that only Col. Davis was to receive a Non-Punitive Letter of Caution vs. the Letter of Censure that he was actually given. It doesn’ mention what they think Gen. Mattis recommended for the other two.
Still it is VERY curious why these Letters of Censure were made public. That alone was “to shape public perception”. It would help if someone would go on record to deny or confirm this allegation.