Seems like playing ball rather than real life would be more fun for a 22-year-old. Nothing against the military; I would say the same of any real-life, only-okay-paying job. My guess is that the percentage of major college athletes that turn down an extra year of playing ball for a regular job is pretty low.
The percentage of people in the military who would call it a regular job is also pretty low. Ditto for those whom the pay is any significant part of the decision matrix.
I can't speak for Marshall Plumlee, but a lot of people join the military because it's what they truly have their heart set on doing. People like to follow their dreams, whether it's the NBA or the service. I personally wouldn't find it the least bit odd or shocking that if he'd found his calling as an Army officer, he wanted to get to it once as soon as he got his degree.