Syncing Music Deletes All Music From iPhone?

I am trying to use iTunes to play a song on my computer. Unfortunately the song is not found anywhere in the iTunes library, despite the phone being synced literally in the last five minutes. The song is on my phone, but not the computer. I looked around and noticed "sync music" was not checked so I turned that option on. I got a warning saying that all songs on my phone would be deleted.

What the **** is that?! The phone is with me 24/7, my computer is not. Therefore everything on the phone: videos, songs, websites, playlists, pictures, etc etc etc. are their most current versions. Basically iTunes is telling me that it is impossible to sync new music from the phone to the computer, despite the fact that literally everyone on the planet uses their phone rather than the computer to make purchases for things to be used on their phone. How do I force iTunes to sync music from the phone to the computer as a one way transaction with no data on the phone being affected in any way whatsoever? This should literally always be the only direction data travels unless restoring the iPhone from a backup.