Uh, why wouldn't they? Calculators check a lot of boxes that militaries care about: cheap, light, compact, durable, easy to manufacture, easy to use. I'd think that they'd be standard issue for any personnel regularly tasked with doing important calculations under logistically difficult scenarios.

Setting that aside... you are aware that modern battle tactics are completely, hopelessly reliant on battery operated devices, right? If a unit gets unintentionally separated from the grid long enough to start running out of power, there will be far greater fish to fry than a few botched arithmatic problems.