To add to this, there is also another line you can draw: do not eat anything which requires killing (not just harvesting from) another living thing. This is basically the line drawn by Jain vegetarians (https://en.wikipedia.org/wiki/Jain_vegetarianism). This allows one to eat milk and dairy but does not allow one to eat root vegetables that require you to kill the entire organism to eat it (and potentially other living beings that live in the soil).

Milk and dairy really don't fit this criterion in any major civilization today. Animals are definitely killed even if they're not consuming them.

I wonder if they can eat the leg of an animal then? Like the joke about the farmer and his favourite pig.

It's very odd to me that not eating mushrooms is part of Jain vegetarianism. If ever there was a case of a fruit that did no harm to harvest, it is your typical farmed mushroom and even many wild ones.

It's not arbitrary; it's based on our best guess of the minimum requirements for sentience.