Good question! The impression I have is that we all, vegans included, draw a line somewhere, and that there's almost never any solid reasoning behind anyone's decision on how to circumscribe his diet. I'd be really interested to hear an avowed vegan explain why it's not okay to kill and eat animals which are capable of nociception, but okay to kill and eat plants which are also so capable.
Nociception implies that the raw sensory data is being processed or encoded in some way. This can't happen without a nervous system. Plants don't have a nervous system, ergo, nociception is not occurring. I am quite convinced that this is consistent with our current understanding of the biology, but would be happy to learn differently.
Another way to approach this ethical dilemma is to proceed from a position of harm minimisation. When you consider energy inputs required to generate plant based calories versus animal based calories, the former is just about always more efficient. To the extent that we consider the moral harm of animal suffering greater or equal to plant suffering, if it exists, we must tend to substitute to plant-based calories to minimise the total suffering we inflict on other life.
