You're leaving for a military commitment, which I am all for. Looking at it from the point of view of the facility, I will be honest and tell you that I don't blame your employer. They would be paying your preceptor and you, investing in training someone as a new nurse who is literally leaving in a few months.
You may be a great employee, but it would be nearly impossible for them to rationalize the staffing costs to upper management. There is no ROI (return on investment). I remember an educator at my hospital saying that the cost to train a new grad was in the tens of thousands. Basically, by the time you are off of orientation, you'd be leaving. (Most acute care settings invest 3 months in orienting new grad nurses)
If you were returning to that specific facility, I believe you would be protected in your new position while completing your military service. I'm sorry that you feel somewhat robbed, and I do not agree with the way they did it but all you can do is push forward. Good luck!