In the event of a flight cancellation though there would be no gate. In the event of a delayed flight then the airline could rely on who actually boarded to decide who was a genuine claimant, but in the event of cancellation, no one would be boarded!
Rerouted people can be traced and see they actually got on their flights, those who make alternative travel plans separate from the airline staff can provide proof as well. Those who abandon their trips totally and go home can ask for a letter from the airline stating they have had their flight cancelled and as a result there is no point in them travelling, which they can use for compensation (most airline ticket desks should provide this on demand).
The ones who can abuse the system would still be those who don't bother turning up at all and simply checked in but decided they didn't want to travel long before the cancellation was announced. They would still have nothing to prove that's what they did, but because they held a boarding card they can still claim compensation.