weekly whatis database and readonly /usr mount

Hello List,

I've installed a testing system with readonly /usr filesystem.
And I was surprised by following weekly report (BTW locate
database rebuilded just fine):
-----
Rebuilding whatis database:
makewhatis: /usr/share/man/whatis.tmp: Read-only file system
makewhatis: /usr/share/openssl/man/whatis.tmp: Read-only file system
-----

Is there any reason for using /usr filesystem istead of /tmp, /var/tmp?

-- 
WBR, Boris Samorodov (bsam)
Research Engineer, http://www.ipt.ru Telephone & Internet SP
FreeBSD Committer, http://www.FreeBSD.org The Power To Serve