The DfT here in the UK stipulate that everyone on an aircraft departing the UK must clear UK security. Hence the transiting SQ pax disembarking and clear security before re-boarding. If, for example, the SQ routes MAN-LHR-SIN then passengers wouldn't need to clear security in LHR because they have already been subjected to the UK security process. For those of you who connect with BA from MAN through LHR T5 you'll know that you don't clear security again because you cleared it in MAN. If you transfer to another LHR terminal you clear security again but presumably that is because you fall into the channel of other transferring pax from destinations outside the UK.
Within Europe there isn't always that stipulation and that is why in the parts of many European airports that service European flights, arriving and departing passengers mix.