User blog comment:Jpx400/Assassin's Creed IV Settings/@comment-4360293-20120915170050/@comment-189.60.7.166-20120916170128

-Spoiler-

If you finish the main storyline, you see that John Marston died, so it makes more sense for his son(who becames playable after his father`s death) to be a Assassin, since we only know he avenged his father.