Earlier quoted context omitted.
The members of the Coast Guard were not conscripted. I don't know if that changes anything for you. But the salient facts are the Coast Guardsmen are not being paid and are not legally allowed to quit. So, according to your previous statement you agree that slavery is an appropriate word in this case. Also, one must contend with the fact that while conscription is not illegal they were paid for their services.
The same concept applies - "national security requires you, a member of the armed forces, to serve, whether you like it or not". I think it's a travesty that the Coast Guard isn't being paid right now, but it appears clear that the courts don't consider such a thing to be slavery in this particular case.
...it'd be slavery if they couldn't quit.
Coast Guardsmen are not being paid and are not allowed to quit. Therefore, according to your statement you think it is slavery.