Not only did Mr. Marple not sit in on the trial of John Ryan Mason, nor hear and see all the evidence, but he is only one person (Sept. 3, On Masons Mistrial).
Our criminal justice system requires that 12 persons , not just one, hear and see all the evidence. All these 12 must agree unanimously. If no unanimous agreement can be made, as in this case, a mistrial is declared. This is how our system works.
Furthermore, I believe that the 12 individuals on this jury came to their decisions without bias toward firemen, as Mr. Marple suggests in his letter.