setting brightness as privileged operation?

Hi,
I have just learned that my Xfce Power Manager cannot manipulate
brightness because I do not have policykit installed on my computer.
There is a reason for that (yeah it depends on systemd which I prefer
not have).

While this is clearly a problem of the Xfce applet I am wondering why
setting the brightness has to be a privileged operation at all. Is there
any strong reason for it or just a general policy that we do not give
world writable files into sysfs?
-- 
Michal Hocko
SUSE Labs