I think it's a bad decision. In order to read the text, I need to zoom in. But I can only zoom in on a part of the page, so I need to scroll sideways back and forth to read the beginning and end of lines in the same paragraph.
It's a bad decision. I should be able to size and flow the page to fit my display, wrapping it appropriately.
Yes, the designer should a layout that does this for mobile devices anyway. But the phone should also make it possible to sensibly read sites that don't.
Setting a static div width should not be undefined behavior for developers. It creates a situation where, if a mobile browser breaks the layout of a site in order to wrap text, the developer then has to go back and create a workaround for a single browser.