I would say that, at first, most people do NOT know that greyed-out boxes can't be changed. They only know after attempting to click on it a few times.

The key is to make it not look like an edit box, then people won't think they can interact with it. You can see it in practice here at HN: edit a submission and you can see that the URL is not in a text-box, so the text can appear like any other text on the page.

In general, making things harder to read can't be good, and should never become a standard.