Why did some Americans consider the draft system unfair?

Vietnam was America's last drafting of young men for the military. And we still had "authorized" ways of buying one's self out of that. If a fellow stayed in college, carried 18 units and a 2.0 average, he recieved a "college deferment." If one was too poor to go or didn't go to college, then obviously he could't recieve a "college deferment", and the army would be sending him a letter.