In ancient times, SSID cannot be disabled mah. So, all computers
using the same access point can "see" the SSID. Then, some people
realized that all the computers in range used the same SSID, so got
light bulb on lah. They figured out how to use the SSID loophole to
go into other people's computer and do things lah. Then,
technicians all headache lor..
So, to save the headache, the technician disabled the SSID feature
lah. Now no one can "see" so no crimes lah. Improved security mah,
especially for public places like SP. Now, we wait to see who is
the first person who can be re-enable the SSID lor..

Read this article
http://netsecurity.about.com/od/quicktip1/qt/qtwifinossid.htm