They kind of have to at some point, otherwise you're using artificial nitrogen to top up the soil and that's not just bad for the soil but is also depleting a limited resource (The nitrogen comes from what IIRC is, a single mine).
I remember reading about some farmers that managed to buck the usual statistics by getting around half a thousand chickens to an acre of farmland -- the chickens acted as a natural form of pest-control for the farmland, and their shit and corpses (when they died) provided a natural source for the NPK needed to feed the soil.
The thing is, vegans forget that plants, etc. have all evolved with death as part of the equation. Animal death is required in some form, to supply the micro-organisms in the soil with the proper nutrients. Anyone who has studied farming and agriculture knows that the ideal isn't to eliminate animal death (something that is pretty much impossible unless you want to destabilize the ecosystem and starve your plants), but try to minimize it and ensure that the death is not wasted.
I think this mistake in understanding goes back to school, where children are taught that if you give plants soil, water and sunlight, they will grow. They could take that opportunity to talk about the NPK cycle, the billions of micro-organisms in the soil that need to be fed to provide the plants with nutrients, the fact that dead animals help feed the plants and the micro-organisms, etc. Not teaching children about this when children are capable of understanding it is a disservice given that for many people that is the only schooling that they will ever have on farming and agriculture.