I really have no idea why Blizzard didn’t just disable all PvP flagging beyond forced toggling and/or revising PvP flagging from guards when you run/get hit by them; it’s rife for abuse and ultimately doesn’t align with their PvP rules for HC servers.
It was either just a lazy decision on their part that they have never bothered to fix or they think it’s healthy gameplay for the HC servers to have certain bottleneck spots where accidental PvP flagging can occur to be abused by other players.