Whoever authorised this ? It is NOT "in the best interests of all travellers" that carriers then cherry-pick their cancellations to keep the most lucrative services going.
The CAA is the UK's National Enforcement Body. It is their remit to enforce the implementation of Regulation 261.
How would you suggest the airlines choose the flights they cancel? Keeping in mind the need to spread out the cancellations over the period in question. Even BA, holding the largest number of slots does not the same number of slots each hour.
Of course the airline tries to cancel the flights with the lighter loads, while considering the loads of returning flights, as this inconveniences the fewest number of passengers. But it is an art, not a science.