They must have done the maths:
1 - The flight must be cheaper than, or competitive with the cost of doing it in two legs, for example with EK over Dubai.
2 - The ticket price must cover the fuel and operating costs. In fact the operating costs will be little more than for Singapore to London, just with the addition of some extra fuel in between.
On my first ever 11.5 hour flight from Cape Town to Amsterdam, my South African agent gave me a little blue pill to help me sleep. She told me to take it, get comfortable, loosen my clothing, attach my belt and I would wake up 7 hours later. "What is it?" I asked. "Rohypnol" came the reply. Thinking this sounded a bit drastic I snapped the tiny pill in half and slipped the unused half in my trouser pocket. Took it and woke up seven hours later feeling a little groggy but a KLM coffee soon sorted that. Generally speaking though I don't think aircrew like drugged passengers in case there's an emergency.