Galloway's purpose in writing the letter was to point out his solution to the problem of colonists resisting the imposition of taxes by the British government. It was the first impulse of what would later be known as the Galloway's Plan of Union, which consisted in creating a parliament of American colonists who would oversee the decisions the British parliament had for the new continent.
In the letter, Galloway recognized the burden of sustaining the colonies as being a reasonable cause for the United Kingdom to tax colonists, but recognized the inability of the later to pay due to the trading restrictions set by the English.
He suggested that if colonists considered the impositions too heavy, they should refer to more rational methods than rioting to let the British understand that they couldn't pay the taxes, for example by sending American representatives to the British parliament.
The evidence is that many of the townsfolk don't want him to actually properly defend Tom Robinson and in all fairness he did not believe he would actually win the case but it was important to try. He defended Tom to the best of his ability as he believes it is the right thing to do and that everyone has a right to a fair trial.
