Why reward unregistered walk-ins for vaccinations?

FIRST and foremost, kudos to the Health Ministry for the efficient vaccination process at various centres.

However, with regard to the AstraZeneca (AZ) opt-in rollout, I understand that many of those who registered failed to show up. This resulted in people crowding AZ vaccination centres with the hope that walk-ins will have a chance to get their dose. And many did indeed succeed.