This question involves a slight misunderstanding of sentencing procedure. During the sentencing phase of a court-martial, if the prosecution wishes to present testimony from the chain of command as aggravating evidence—for example, a commander testifying about the negative impact of the accused’s offense on the unit—the defense has an absolute right to cross-examine that commander. The constitutional right to confrontation applies during sentencing.
However, the defense often chooses to submit written “character letters” from supportive members of the chain of command as mitigating evidence. In this case, because the defense is voluntarily submitting the written statement, they are waiving the right to have that person testify and be cross-examined. The written letter, which is hearsay, is admitted for the limited purpose of mitigation. The prosecutor can object to the letter, but judges will almost always allow it.
The panel members will then weigh the written statement accordingly. They will consider the author’s rank, their relationship to the accused, and the content of the letter. A detailed, heartfelt letter from a direct supervisor can be very persuasive. The defense attorney will use these letters to argue that the accused is a person of good character who has value to the service, despite their conviction. The key is that if the evidence is live testimony it is subject to cross-examination; if it is a written letter submitted by the defense, it is not.