By doing this, the remaining trees have more resources available to them, like water, nutrients, and sunlight. This improves tree growth and vigor and makes the trees resistant to bark beetle attack. The key to reducing the potential for future western spruce budworm and tussock moth outbreaks is active management of stands, but at a much larger scale. To be effective, though, this needs to occur over large watershed and landscapes. Salvage logging is unnecessary if you thin appropriately.
Salvage is kind of like cleaning up after the war and the bodies are lying around.
What you want to do is something before the war starts to prevent the war. Private industry has done fairly well in managing their land for density because they haven't had the restrictions placed on them that the public land agencies have had. With those kinds of restrictions, it's difficult to manage forests to bring those densities down. If you have density-dependent species like spotted owl, that's in direct conflict with density as far as growing healthy trees. Salvage logging is a reactive rather than a proactive measure.