My wife and I (while she was alive) have always brought up our kids to have a certain respect for authority, while at the same time having a jaundiced view of their motives sometimes (what can I say we're of a liberal bent). My daughter's mid way through a mechanical engineering degree (she was stripping and rebuilding car engines from the age of 14) and has told me she wants to enlist in the navy in an engineering capacity when she graduates (apparently they've said they'll pay for her to do a doctorate). Part of me wants to say "You want to do what?" in a loud voice at her, followed by "You do know people shot at people in the navy too?" So the question is how irrational am I being? and how best should I play it?
Well, the truth is, joining the military for the sole purpose of having your school paid for isn't a great idea unless the plan is to have a career in the military. That's really what it's intended for, it's just that the military started to advertise it more to get more people to join. I might point out to her that most PhD students get funding through the school they attend, so if she's a good student, she probably won't have to worry about it. And I don't think you're being unreasonable, I think you're being human. I think unreasonable would be to attack her or disrespect her for it. You're simply being concerned. I would be. It is her decision, but that doesn't mean you can't give her your opinion on it. She'd probably want it. It's not an easy decision to make alone. You just have to respect her for the decision she ultimately makes.
Being fair, I don't think she's just doing it for the money. She's quite the little engineer and views it as a way of working to her strengths. It still freaks me out some though.