There is a huge difference in stealing someones kills and shoot a harvester of another player who start shooting it first.
You who shoot at harvesters what another player has started to shoot at, you obviously don't realize you can cause the dead of that pilot as the interceptors is going after the one that shot first and not after you.
It is not correct either to cause another players dead by insisting shooting at the harvester even if they ask you politely to stop.
Several players has become a pod on low level harvesters, just because of this reason, it's not very nice when a player is trying to help someone who is lower to grind, then someone comes along, and start to shoot without stop on the harvester what he/she was shooting, and then the first gets a huge amount of interceptors attacking him/her and they are not prepared for it, and they might systematically shoot harvesters, then the landing Interceptors, and only after the Harvester again cause they cannot deal with more at time.
Don't always take the issue that the person is "owning" the harvester, but they are just trying to keep themselves alive.
I believe it's not correct to cause other players dead if you can avoid it, it is pure evil to continue in these cases.