This is a follow-up question to "why-one's-daughter-is-not-mentioned-in-arayos-in-the-torah" (please read before continuing).
Both the Gemmorah and Rambam agree that the prohibition of one's daughter comes from a Drashah (either Kal Vachomer or Gzerah Shavah).
In his Introduction (Shoresh 2) Rambam specifies that:
שאין ראוי למנות כל מה שלמדים באחת משלש עשרה מדות שהתורה נדרשת בהן או מרבוי:
Do not include laws which are derived from one of the Thirteen Principles of Torah Exegesis (CHabad.org).
Those two look like a contradiction not only to me by to RambaN also.
Why does Rambam override his own rule especially in this prohibition?