The few examples in my life of family and close friends who volunteered say it's a combination of things, it's not either money or something else, even in individual cases, but money and the job situation is definitely a factor. Let's see, my brother joined up (navy-Vietnam era) because he thought it would straighten his life out and make a man of him (that's what everyone said, including our dad), and get him out of boring jobs. He also thought he'd learn things and see the world. When he was given an involuntary medical discharge, far short of his career goal of 20 years, he was heartbroken.
My nephew joined at 30 because he was getting deeper and deeper into debt from his school loans and credit card and even though he was working a job he liked he couldn't get out from under the bills (this was the mid-90s). The military said they'd pay them off, and they did. It was really the only way he saw to get out from under them. (This may be more common these days, a friend's brother in law joined up cause he couldn't find a job, was fired by Clear Channel and couldn't get another radio job because they're all Clear Channel where he lives--3 small kids, wife is low-paid--and he is now being shipped off to the Middle East.)
A close friend joined (Vietnam era) because he'd 'be a man' and to fight commies. He also had a criminal charge against him and they said they'd make it go away if he joined up. He now sees it differently, but it was desire to make something of himself and to contribute to what he thought was right, and definitely he wasn't going to college cause there was no money. Later he did on the GI Bill.
My dad volunteered (Army Air Corps--WW II era but before Pearl Harbor) because he thought that of all the jobs he surveyed, civilian airline pilots would have the most leisure time, so he wanted to learn to be a pilot, and the way you did that was join the military. At the time he had no idea, he says, that the US would join the war, so he was somewhat surprised to find himself lead navigator for a B-17 squadron in England.
Another friend joined because he wanted to be doctor and it was the only way to pay for medical school. You get three years active duty, plus five years during which you can be called if needed (Individual Ready Reserve). He was sent to Iraq in 2003.
Another friend joined the Army (late '70s) because she says it was better and more interesting than the factory jobs she was doing and she wanted to see the world, get out of her small town, not end up pregnant and married to someone she didn't like, as most of women in her family were.
So of this small sample: paid training you would not be able to get otherwise, debt, unemployment, lack of opportunities, patriotism, trying to become a 'man,' desire for meaning, desire to see other places, to make something of yourself, to make something happen. The question is, if there were jobs where you could get all these--free job training, free college or professional school, jobs that pay well and have meaning, travel--but without all the killing or getting shot at, would the military be as attractive? Part of the evidence that people are weighing financial factors is that the military's offering scholarships and paying off school loans. If money were not a factor for their recruits, they wouldn't go to the expense.
I agree it's not so much a poverty draft, that's not right, it's working-class draft, an unemployment draft, a no-paid college draft, a job-discrimination draft, a meaningless work draft--helped along by some ideology. (Sometimes it's not what you think, though. Black Panther (and Vietnam war vet) Geronimo Pratt said in a speech here that he had joined the military because he was designated by his small Louisiana town to be one of the ones who went into the military to learn weapons handling and warfare so that he could come back and defend his community against the racists. I guess you could call that job training.)
Also, this exposes a weakness of talking about U.S. society by income ('poor,' 'middle income') rather than by relationship to those who control the means of our survival. If you just say poverty, then it becomes mysterious and desperate and otherish. If you look at it by class, the goal of joining the military is at least partly to get some power over your situation, to have some say it what happens in your life. It doesn't work, in many cases, but it's a goal.
Jenny Brown