Stop songs deleted from iPod from reappearing after syncing?

When apps sync with iOS devices, and I delete an app from the iOS device, I can sync the iOS device with iTunes and the app does not reinstall onto the iOS device.

When music and movies sync with iOS devices, however, and I delete, say, a song from the iOS device, whenever I sync the iOS device with iTunes, the song is recopied onto the iOS device.

Why is iTunes so insistent on keeping music and movies so exactly the same between computer and iPod, but far more liberal for apps? Is there a way to make music and movies sync more like apps?