I think a lot of it has to do with the goal of the game (which of course, is something people do not necessarily agree on).
To extend the dodgeball analogy (although it's not really a good one, since it seems to be constructed with the assumption of "the purpose of the game is to blow up each other's ships, all other activities are to support buying ships to blow up"), the goal of dodgeball is to tag members of the opposing team with the ball to get them out, not to inflict as much pain as possible on your opponents. While certain situations or vigorous attempts to achieve a tag may inflict pain as a side-effect, most people generally agree that you shouldn't be playing to maximize pain inflicted and so generally avoid aiming for areas such as the face or groin.
Unfortunately, some immature people who play dodgeball set inflicting maximum pain as their own goal, regardless of what everyone else is going in expecting. Anyone here who's had a PE class has probably met at least one person like this. The people who deliberately aim for the face and groin, and seem to be more concerned with breaking noses than scoring points. They'll insist that it's just legitimate gameplay on the one hand, while gloating about inflicting injury on the other. Those are greifers. Even though strictly speaking there's no practical way to stop them playing like that, especially in a PE class where participation is mandatory and the other players can't just refuse to play with that person, playing that way still makes them a massive jerks.
In Elite, to most players the goals of the game are to earn credits, buy ships to fly around in, and support their favorite factions. While some of these activities may involve blowing up ships, even other players, that's not the primary goal. "How many helpless Type 6s and Haulers I've murdered" is generally not considered an acceptable performance metric in this game. Piracy is considered at least somewhat legitimate because in theory it earns credits (or at least we like to pretend it does), thus actually contributing to the main goal of the game. Blowing the target up when you're done is frowned upon because that's just unnecessary ery.
Straight-up ganking draws a lot of ire because it doesn't even try to work toward the main goal of the game. It's more along the lines of "Yes, you CAN take some lava buckets in Minecraft and burn the whole map to the ground, but you really shouldn't." There's no reason to gank a trader except to be a jerk on the internet, so of course doing that makes you a jerk. Or to go back to the dodgeball analogy, it's like continuing to pummel someone who is already out. You don't score any points for hitting someone who is already out, and no there's no such thing as a bonus for bruises. At that point the pretext of "just playing the game" is flimsy at best, really you're just being a jerk.
(edit: it's also really hard to have an adequate discussion about greifing when all but the mildest words for "people being mean" are on the filter list...)