Comment by SilasX

Correct, but I agree with the parent that this is a dubious case to apply that reasoning.

To make it clearer, imagine another context: "It's dangerous for a passenger to have a gun on board. Therefore, we're strictly limiting passengers to only two guns."

Like, no. The relevant sad case is present with one gun just as with two.

Of course, what complicates it is that these power banks present a small but relevant risk of burning and killing everyone on board. So yeah, you might be below the risk threshold if everyone brought two, but not three. So it's not inherently a stupid idea, but requires a really precise risk calculation to justify that figure.