You could definitely ruin a concert by projecting offensive symbols or messages. Especially since if it's done well, most concertgoers would initially assume that it's part of the show.
Why huge? Something couple times stronger than a TV remote should be fine, if focused. Unless they purposefully put a powerful IR attenuator in front of the receiver, as a security measure so that a would-be attacker would really need a a "huge IR spotlight".
Because distance and reflection are the attenuators here. A stadium-sized group of kids aren't very good at pointing your receivers at your rogue transmitter.
Even if it was possible to propagate with the bands themselve (I think the vendor’s website mentions there is a Bluetooth version, maybe you could with that), you still have the problem of not knowing where any are, so if you actually want to show anything meaningful on a group of these, you can’t really. You can just make annoying flashes.
