That share is visible (browseable), writeable (read only = no) and accessible by anyone from Samba's point of view (guest -- ok). But in Linux filesystem it is owned by root so in Linux filesystem it is only writeable by root.
Samba looks at both constraints (ok to write in Samba but not ok to write in Linux) and imposes the stricter limitation. So the result is that it's not writeable.

To make it writeable you change the Linux permissions from rwxr-xr-x (which prohibits anyone but root) to rwxrwxrwx (which allows the world). Then Samba says "ok for the world" and Linux says "ok for the world" and thus it does become "ok for the world".

Phew! :blink: