Additionally, it's an all-volunteer force. That doesn't mean people don't sacrifice to join up. For most career fields, we make considerably less than the civilian sector, and live with a more difficult and dangerous lifestyle.
For me personally, I've missed the majority of the past year because of deployments and trips. I'm not doing it for the $. That would make me an idiot. It's just something I feel compelled to do for a host of non-material reasons.
Donating to groups like Wounded Warriors is a very worthy use of your charity.