I think I'll open OnyX and see if I can remove that option.
Hah! I opened Onyx and searched until I hit Parameters > Finder > Finder Menu > Show Quit Finder, which was enabled (apparently by default); I unchecked/disabled the feature, and it disappeared from the menu. I then tried the Quit command while in Finder, and it didn't quit Finder (as one would hope). Amen to all of that.