Except it never state it is a natural weapon in the description, it say "gains the ability to use her hair as a weapon. This functions as a primary natural attack with a reach of 5 feet." I understand how it can be confusing but it's stated it function as a primary natural attack, not a natural attack or a natural weapon, hence it can't be used in as a secondary attack. Believing it's a natural weapon is interpretation. In every other descriptions of abilities giving natural attack it's always implied that it can be used either way, look for metallic wings, lesser fiend totam, even the claw of the draconic bloodline specify it function as a natural weapon, not a attack.
