'They', the vegans, are not homogenous. There is no rule book, stating what they are and are not 'allowed', nor a set of approved reasons.
People eat what they choose to. Many apply ethical ideas to these choices. Vegans are people whose choices, ethical or otherwise, have taken them to a particular dietary point. People become and remain vegan for a whole swathe of different reasons.
So if you'd like one vegan's take on why they eat what they eat and where exactly they draw the line, you could ask one vegan, and that's what you'd get.