This was not a demonstration. It was a rally, where people had to leave these things outside with nowhere to dispose of them.
IMO, the distinction doesn’t make much difference to me. If I had to leave it out there to go inside, I would have just retrieved it when I came out.
But then again, I do have a thing about littering. Perhaps it is true that many people wouldn’t do that.
Anyway, I think it is illustrative that the Left has expended some time and print space to highlighting this, so...it just reinforces that this is something they are always guilty of, being as good at projection as they are.