US celebs (or even common folks) traveling within USA don't need to pass customs.
Anyway, the power is discretionary, usually randomly exercised based on hunches or specifically targetted based on tip-offs. Searching Bill Gates, e.g. when he's re-entering US from abroad, is insane, isn't it? This guy got enough money to own poppy field as large as a mid-sized state or hire an army the size of a mid-power country to traffick drug for him if he wants to. Hell, this guy is even too geeky to know how to smoke a cigarette.
Look back at the McCartney example in Japan. They based their actions on intelligence and information. I don't know this Kenny Bee well but I suspect there's some background info that prompted Singapore customs action.