I disagree with this, mainly because all graves spawn ghosts when broken, including those dropped by NPCs. If what you're wanting comes true, then the player has to mine a grave and fight an annoying ghost every single time an NPC dies. Since you can lose a lot of NPCs from a bad invasion or boss battle, this will become very annoying. Even if the player doesn't care about their town's looks, or doesn't want to fight ghosts, ignoring the graves will eventually turn their town into a graveyard once enough die, forcing them to fight ghosts anyway.
While not having to die to create a graveyard in softcore/mediumcore would be nice, the change you're suggesting would anger the majority of players who don't care about creating a graveyard biome. Now, you're probably wondering "what if NPC graves didn't spawn ghosts?" Well, the trouble is that this is how graves are coded in Terraria. Graves function like signs - the only thing they track is what is written on them, which defaults to the death message. Making graves track who died adds requires entirely new code, since nothing else does this.
As for "forcing" the player to die, I'm pretty sure most people who choose softcore/mediumcore are okay with dying in the first place.
This isn't a huge issue for mediumcore characters. They way they handle graveyard building is like softcore characters with an extra step.
If you want to farm gravestones, just put your money (softcore) and inventory (mediumcore) inside a chest and then kill your character.