13th amendment makes slave labor legal for those that are imprisoned. And do you make the same argument for community service?

I think it's properly conscription yet doesn't quite become as bad as prison labor. They'd have better personal security, more freedom, get to choose and keep their own stuff, and access to the world. Compared to prison, a military base is a spa.

The 13th Amendment does explicitly carve out an exception for this. Not that I approve, but it's perfectly constitutional.

If you can't do the time don't do the crime. Anyway, I'm sure he'd rather do that than sit in gen-pop worrying about getting shanked or having to join a prison gang.