Preston's innocence is proven by the testimony of James Woodall because he stated someone else came along, touched the soldier's backs and told them to fire. According to James, "the captain, after, seemed shocked and looked upon the soldiers". He then said he was very certain Preston did not say fire. He also mentioned how sticks and ice were being thrown at the soldiers and knocked one of them down. Soon after, that soldier took his gun and fired directly into the crowd. He fired without Preston's orders which already proves how the others and himself were not obeying Preston in the first place . With this I can conclude the soldiers were confused and overwhelmed with the situation, or they had been given orders from someone else, because the soldiers would have been positive of Preston's orders and would have fired upon them.
Here is how the soldiers would have looked like