Singapore isn't exactly huge. What's to stop someone from setting up Wimax in Johor Bahru (just across the border in Malaysia) and broadcasting the signal into Singapore?
I don't think this is practical - even if the Malaysians allowed it, Johor Bahru is north of Singapore and the main population centre and business district of the island is along the south coast.
Signals at such high frequencies simply wouldn't penetrate the extremely dense urban landscape of Singapore other than along a narrow northern coastal strip.
A signal like that would be very easy to jam for a government if they chose to do so. Encryption (if you can connect to an outside host) or a channel that is harder to jam (satelite, wires, laser beams, etc.) are the only real options.