I don’t think the reason they’re being used is relevant to their problem though. “Think like an attacker” wins the day here: as an attacker, I don’t care what it’s meant for, only how I can use it to my advantage. If it’s something they observed as a problem, I understand why they would want to stop it.
As for “-”, yeah, I don’t have a particularly good explanation for that one except the assumption that it’s something similar to + addressing on a different service.