What logic? Why would you expect there to be any logic in it? This is aviation security we're talking about, after all!

No, there's no logic in it -- not as a long-term defense, anyway.

If there is specific evidence that the attackers were specifically planning to use camera flashes rigged-up as a makeshift detonator (say), then it might make sense to ban camera flashes for a few days as a short-term measure. For instance, such a step might cause any would-be terrorists to postpone their attack and come up with a new plan rather than trying to adjust on the fly.

However, as a long-term defense, it is clear that this is an arms race where the terrorist will always win. The defenders have to make the first move (i.e., specifying what is banned). The terrorists can then respond to identify and exploit any gaps left in the defenders' policies. The terrorists will always win that battle; there is no way to ban everything that could be used as a detonator.

So while it might (arguably) make sense to ban this stuff for a few days for ex-London flights based on very specific intelligence, it would make no sense whatsoever to maintain this as a permanent policy.