Which makes perfect sense. Any weapon leaves some trace (even if only new theory as to what is possible), so its use against a party that does not have that technology but is capable of understanding the technology
at some level will always give information to the enemy.
Using a modern missile against an indigenous people will only impart that you are capable of that type of attach.
Using a modern missile against WWII Germany would likely quickly result in refinements to their V2 Rocket program, given enough remains to study.
Using a modern missile against Vietnam era USA would likely result in advancements in miniaturization and computation, given enough remains (even if they did not have the resources/facilities to capitalize on some aspects of those for years, I think it's likely it would advance the fields by a least a few years).
One of the biggest advantages the Allies had in WWII was that they had cracked the "uncrackable" Axis encryption. Even though they were able to decipher enemy messages, they often didn't act on that information because that would tip their hand. The strategic value of reading the enemies messages is enormous when the enemy doesn't know you can do it, and much less so, and possibly even negative when they do know.