Can someone explain to me why TextEdit doesn’t automatically disregard rich text formating in HTML files?

I wanted to make a few simple changes to some HTML files so I grabbed the closest and simplest tool at hand on OS X, TextEdit.  After a few minutes editing, I lost about half an hour wondering why Firefox rendered everything as text, including all the HTML tags.  I thought maybe I had lost my mind, until I discovered that TextEdit had inserted rich text formating and that I had to explicitly tell it not to with a config option.

I think it follows that if I’m editing an HTML file, at some point I’m going to open it in a browser, so don’t put in the rich text formatting.

Apple, you should fix this.  It isn’t like you to overlook the small details.