Note that in verse 10 the reason had become a prohibited oath. This tells us that we have a problem in the text. There is a very clear model, and the reference to the very clear model is inaccurate. What is happening here?
The best explanation comes in the nature of the translation, where Joseph Smith drew on his understanding as he translated the material before him. He understands that there is a reference to prohibited oaths, but he is not reading those oaths, and therefore mistakenly places the reason as the oath.