Yes, it’s because the developers think people expect a “flash” feature, and they think they can have it send light only when it makes sense.
You are correct that it would be a nice alternative feature, to be able to illuminate while the human on choosing to take the picture.
Sadly, even if I turn on the camera’s light, as soon as I activate (or task switch to) the camera, my “mobile device / smart(ass) phone” automatically turns off the light. I doubt there is a real good reason for that.
I have seen some people say there are some camera-phones that don’t turn off the flashlight. Also it seems entirely possible to write a program that will do so. There should be a setting for it, somewhere . . . but that doesn’t mean there is.