They are no longer caught up as we are with the cycle of Samsara. For them even dying, sickness, and injury is not dukkha or unsatisfactoriness. So they don’t have to defend themselves for their own sake.

But they may well do it for the benefit of others. First, if they die, that means we lose our connection with them, and they can’t help us in the same way. And if damaged that may impact on how they can help others in practical ways. It’s also natural, our bodies protect themselves anyway. If you get sick, your body will protect itself against the invading germs. If your skin gets cut, then it will protect itself by healing its skin to prevent harmful organisms from entering the skin.

As well as that though, if someone harms a Buddha then it is cutting off their connection to enlightenment in a rather strong way. It’s not easy to harm a Buddha because they are so open to everything around them, at least according to the stories in the sutras and elsewhere, there are stories in the sutras of various people and beings trying to harm the Buddha and they couldn’t succeed. But if you do, then it is cutting off your connection with them, and more generally with the openness of enlightenment itself.

Then, there’s the question, is someone a Buddha or not? You can come to see enlightened activity in others and for a Buddha then according to some of the teachings in some of the schools they see all beings as Buddha. So it’s not like there is an objective test to see if someone is Buddha.

Also - Buddhas because they are not caught up in the illusions of Samsara - they can do things that benefit us without any worries about how it will reflect back on them. They don’t care if everyone in the world thinks they are a terrible person. Nor do they care if everyone decides to give them a big medal for bravery and courage and generosity. So, that’s just not a motivation, so if defending themselves, or some other forceful action is what their compassion and openness and wisdom leads them to do, they just do it.

Also enlightened beings continue to have a history, a past. Buddha himself was born in a particular place, became enlightened at a particular spot, grew old, got sick, died. So there is a person, as we understand it and them becoming enlightened didn’t change that. So in that sense there is a person to be injured, to die. There is something to protect there. So you can’t say that because they have realized non self, that there is nothing left to protect. In a way there is, in a wya there isn’t.

Also as for their self not existing, well nor does yours nor mine, in the sense in which you can come to see the truth of non self. Buddhas don’t achieve anything or go anywhere when they reach enlightenment. They just see a truth that is open for all of us to see. They don’t destroy their self, because it was never there to be destroyed. They don’t build up an illusion of non self either. Some meditators may do that They may get so caught up in trying to see non self that they build up an idea of “non self” which they then have to defend, for instance they might try to defend that idea that they have realized non self, by not defending themselves against aggression. But an enlightened person doesn’t have an idea of non self to defend either.

So you can actually equally ask the question of yourself and myself. Why do I defend myself when my self doesn’t really exist in the sense that I think it does? Why do I always act as if it does?

If this is puzzling for a Buddha, then it is equally puzzling for us. And, though it may seem that I am doing it for entirely selfish reasons, it might also be that if looked at in just a slightly different way that it is enlightened activity. As I’ve been taught it by my teachers, it’s such a tiny change, really nothing changed at all. Like waking from a dream. Our awakened mind is plain to see all the time and it’s because it is so clear and obvious, if we could but see it, that we miss it, over and over, never see it.

Oh, and in case this sounds like a Vulcan like coldness, well, no, as described in the teachings and stories to help us understand enlightenment, Buddhas are immediate in their actions. They don’t have to do long trains of rational thought to decide what to do, or stamp down on bothersome emotions in every moment, not as it is understood in the Buddhist teachings in the sutra traditions.

In every moment they respond appropriately, that’s their wisdom, what’s sometimes inaccurately translated as omniscience. It doesn’t mean that they know e.g. the position of every rock on Mars :). Buddha often showed that he didn’t know things in that sense, that his followers did things he hadn’t expected, events in the world happened that he couldn’t have predicted. For instance the story of the monk who he sent off to get robes so that he could ordain him, and who was gored by a cow, and never came back to him (that’s Pukkusāti in the Dhātuvibhanga Sutta). But each moment has the wisdom and understanding to respond to it built right into it. So they may do forceful action, spontaneously and surprisingly. They have something equivalent to our strong anger, but instead of a fierce rage, and the other negative emotions, without the strong narrow focus we have in every moment, on the self that isn’t actually there, they have a clear and strong love, compassion and other positive qualities. The full range of things we can do, they can also, but without that narrow focus, it becomes positive, open, compassionate, loving and wise conduct.