Why does reapers bones count towards pirate legend status?

They are enemies, but mechanically its just set up for any 3 factions, i dont think in lore there are levels in the factions to determine your character as a legend, but more just your deeds on the seas and making a name for your self, its just that reaching level 50 across any 3 factions is the closest way to represent making a name for your self on the seas regardless of which faction it is.