Joseph,
1. Who says that they have enough? Moreover, the army needs precisely the type of soldiers yeshiva men would be – analytical types who can work in Intelligence. As for Shevet Levi, Rambam also says that in a milchemet mitzva (such as saving Jews) EVERYBODY goes (Hilchot Melachim 5:1 and 7:4).
2. If someone is not really learning but just goofing off besides the fact that he is robbing those who pay for his upkeep he is taking a deferment under false pretenses, which is geneivat daat.
3. What about having them do their service in an army bet midrash, and state explicitly that their learning is for the success of the troops as well as saying the misheberachs for the State (which is paying them so they should show some gratitude)and the soldiers?