I am no expert on the CW from what I read there are several reasons given why Southeners joined the Union Army. The main one given is a sociological one in that lower class small farmers in states such has Al especially in the hill country were looked down upon by the rich slave owners and while not interested in fighting in the CW pe se where angered by the CSA conscription act of 1862 and figured if they had to fight they might has well go with the side that wasn't trying to conscript them and seek revenge on the slave owneres who they never liked much anyway. The second reason was (mainly in Tenn and to a lesser extent in NC and Al) was loyalty to "the old flag". The third reason was that in Camp Douglas Il many CSA prisoners were given the charming choice of either risking close to certain death from malnutrition and disease or going the Rocky Mountains and fighting Indians and freezing weather. Many CSA "volunteers " took their chances out west.
On the other hand I have learned that in Tenn some slave owners were officers in the Union Army. Did any Al slave owners joined the Union Army? The sociological reason makes intuitive sense but maybe its not the complete reason that many Southern white males (approx 100k) joined the Union Army or associated paramilitary groups?
On the subject of conscription a broad one would be did conscription help the South more then it hurt it taking into account those Southeners who joined the Union Army? To be fair conscrption by the Union in 1863 was not an overwhelming success to say the least.