They're literally serving the content with a text/plain media type.

If your browser is rendering plaintext documents in a way that's unreadable, that's a failure of your web browser to serve as an effective user agent for your needs.

(People shoot down the analogous argument for changing the base formatting of text/html, because changing the base UA styles would throw brittle old stylesheets out of whack. But plaintext doesn't have stylesheets that could be thrown out-of-whack.)