To clarify, I don't see "combat tourism" as the only explanation. I think the military can provide a useful hand up to those in difficult situations. We just need to be clear that that is essentially a selfish argument, not one for service.
Regarding the scenario you've laid out: in general, people will do what they need to do for family survival, me included. But, if I steal or I kill to save my family you are right to criticise me.
Additionally, how is that any different from the choices in front of the people whose countries we go and invade?
In Restreppo, e.g. the captain lectures villagers after an Apache strike kills and maims a number of civilians. It was basically, why do you allow the Taliban to shoot from your village, so that I have to come and kill your sons?
Exactly the same argument applies in reverse. If you voluntarily join the aggressive American military machine, you're no better than the villagers who we kill for failing to root out the Taliban.
If you're a Sunni in Iraq and you're being massacred by Shia death-squads, then why is it wrong to support this ISIL gang who promise to protect you? You see where this argument leads?
I have sympathy for US soldiers, but I have more sympathy for civilians facing worse choices in the countries that we tend to invade or intervene in.
There are ways to serve that do not make you part of a machine that has historically been used to commit mayhem and murder on a vast scale.
EDIT> None of the above means that I think it is justifiable to claw back these enlistment bonuses. A promise was made to these enlistees, and it should be honoured. Is there an estimate of the total cost to forgive all of these repayments?