I've just noticed this bug.
IMHO, /usr/share/nginx/www/ should never be used to actually serve files. To serve static files, the document root should be changed. Somewhere under /srv is suitable under the FHS.
It would be nice if this were documented better somehow (I don't know how), but this is a deliberate decision on the part of the Debian nginx maintainers. See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=730382 for some background and the current debate (which I note already links here as an example of how things go wrong).
I'm all for changing the default document root to /var/www/html/ like Apache to avoid this issue, but I think that this change is still up for debate in Debian. I'm neutral on whether Ubuntu should unilaterally make this change outside of Debian. On one hand I prefer not to diverge, but the existence of this bug does demonstrate that this is a real issue for real Ubuntu users. Apache's way seems to do better and is decades old.