阿里拒绝应征的原因，他的原话是

“My conscience won’t let me go shoot my brother, or some darker people, or some poor hungry people in the mud for big powerful America,” he had explained two years earlier. “And shoot them for what? They never called me nigger, they never lynched me, they didn’t put no dogs on me, they didn’t rob me of my nationality, rape and kill my mother and father. … Shoot them for what? How can I shoot them poor people? Just take me to jail.”

克林顿本来也是想做conscience objector, 但为了政治生涯登记了，这个在他给Holmes 上校的信里写得很明白。

https://www.pbs.org/wgbh/pages/frontline/shows/clinton/etc/draftletter.html

不管怎样，的确是可以拒绝的。