[phantomathkg]

I think the reason is, like you said, it is a transit hub.

So passenger flying out of Singapore can mix with people in transit. Therefore, putting the scanner before boarding prevent a possible terrorist attack when someone smuggle in explosive or what not from Singapore or from transit flight.

[yardstick]

In Dubai, which is a massive transit hub, they do the security check upon arrival, before you enter the departures area. They still do some lighter spot checks at the gates, but liquids allowed.

Heathrow as a major transit hub also doesn’t do security checks at the gate, rather it’s when you enter airside. Far better.

Singapore and KL airports are the odd ones out. I suspect because they were built before the security rules required better separation of departing and arriving passengers, and retrofitting the added security paths made less sense financially than just doing checks at every gate.

[jrootabega]

Kinda like needing to enter your password again to change your account settings if you're already logged in. (I don't necessarily think all these rules are good and just, but if they were, it would make sense to have some redundancy.)

[msm_]

This is not how it works in most of the world. Usually you don't have to go through security when you transit, only when you arrive at the airport. The only time when I had a second security screening during transit was in the US.