Again, the Half-Orc entry in the PHB page 19, under "Orc Blood" it says:
Then, the Orc Double Axes entry in the PHB page 99:
Note the title of the weapon is directly incorporated into the description. This isn't fluff in my opinion. I believe it is a special orc weapon. If it is a special orc weapon, the half-orc is proficient in it because of their "Orc Blood" bonus.
I can understand people not agreeing with this interpretation (I myself am playing a bit of the devil's advocate to lend some support to the original poster to this thread). However, I think the argument is strong enough to warrant more than just "It doesn't say that, so it must not be so". If "special orc weapons" means something, then we should try to determine what is meant. Simply dismissing the "Orc-Blood" bonus without analysis is, in my opinion, selling the half-orc short.