There could be two possibilities--one did not like the manner in which the impeachment process was conducted...another and equally likely is that he was senator from the same state as Johnson and served under him there.
Fowler was born in Steubenville, Ohio. He graduated from Grove Academy in that city and subsequently from Franklin College in New Athens, Ohio in 1843. He taught school in Shelby County, Kentucky in 1844. He served as a professor of mathematics at Franklin College in Davidson County, Tennessee from 1845 to 1849. He studied law in Bowling Green, Kentucky and was subsequently admitted to the bar and practiced in Tennessee until 1861. He also served as president of Howard Female College in Gallatin, Tennessee from 1856 to 1861.
Fowler was an ardent Unionist. He served as State Comptroller of Tennessee from 1862 until 1865. The official Tennessee Blue Book states that the holder of that office during this period was "Joseph S. Foster". It is likely that the Biographical Directory is the correct source; the Confederate state government of Tennessee was never very effective and largely disfunctional due to the early loss of much Tennessee territory to Union forces by the Confederates and also because neither the Confederacy nor slavery had ever had much public support in East Tennessee. Most of the state was under the control of the Union military government of Abraham Lincoln's appointed governor, Andrew Johnson, for most of the duration of the American Civil War; his government was fairly functional and it is likely that Fowler served this regime as Comptroller and that the Blue Book records his name erroneously.
In 1866 Tennessee became the first former Confederate state to be readmitted to the Union; the Tennessee General Assembly elected Fowler to the Senate, where his service began on July 24, 1866. Fowler became a part of the majority Republican caucus. In the 40th Congress he served as chairman of the Committee on Engrossed Bills.
During President Andrew Johnson's impeachment trial, Fowler broke party ranks, along with six other Republican senators, and in a courageous act of political suicide, voted for acquittal. These seven Republican senators were disturbed by how the proceedings had been manipulated in order to give a one-sided presentation of the evidence. Senators William Pitt Fessenden, Joseph S. Fowler, James W. Grimes, John B. Henderson, Lyman Trumbull, Peter G. Van Winkle , and Edmund G. Ross of Kansas, who provided the decisive vote , defied their party and public opinion and voted against impeachment.