The idea that the soldiers gave thrives wives an issuance of the writ of divorce prior to going to war is mentioned in Sanhedrin 107b. So the source of this story is Talmudic, not Biblical. Take it for what we will.

Even if a soldier gave his wife a get (The divorce contract) that she could use in the event of his death, it doesn't justify David's actions. Even if Bathsheba was destined to give birth to Solomon with David as the father, there would have been another way to do it other than the way David made it happen. (Properly, perhaps Uriah would have died naturally, rather than through David's interventions). A get or not, it was a very serious sin the way he did it.