Titan Mauler Question - Large Greataxe is a siege weapon?

[Kizan]

As a 10th level Barb Titan Mauler I was going to use a Large Greataxe. I create the Large Greataxe and it shows up in the list of weapons now as a Siege weapon. I purchase it and it shows up on my list of weapons, but I can't equip it. I checked a little and the Nodachi does this also. I haven't checked the other two-handed weapons.

I must be missing something. Is there a rule or something that I didn't see? I think this should be legit.

Thanks,

Kizan