I don't think it's Singapore specifically, seems like all flights incoming to the US always have to pass through some sort of secondary security. For example, M

I don't think it's Singapore specifically, seems like all flights incoming to the US always have to pass through some sort of secondary security. For example, MUC->DUS->JFK required additional security when boarding the JFK bound plane (and it was additional even for those entering the airport at DUS). JFK->SIN though we could get off and didn't have to worry about any security or immigration.