Then they can earn positive reputation amongst the community as traders to avoid being ganked by some. This game is just as much a social game, than it is a sandbox. Political silver tongue or even just being a neutral fair trader, can get you pretty far amongst groups.
Ichorus, Ruben and Kenvic1(Avatar) are some well known traders that people come across and won't kill, due to respect.
Recently, Me and my group were farming the forest troll, An Artisans group rolled up and could of ganked us while we were fighting the troll. They offered to help kill it instead, which we declined. We could of turned on them, but we didn't due to the fact they respected our space. The fact Artisans didn't go against their word and proved they were a true Neutral trade guild, made me decide to let them pass and not kill them, if I come across them in the world, unless they attack me or my group.
Deleting items in PvP situations, affects the risk vs reward element and also negatively impacts supply and demand. Understandable if the game allows them to do it, they will. But it's still a scummy thing to do(like combat logging is) and should be patched.