I have a lot of anarcho-capitalist friends. Mostly we get on fine, but there's a limit to our agreement: they say all government is bad, and I say, yeah, but I'm okay with some anyway. Here's why we disagree:
Everyone accepts that people do good things sometimes, and bad things other times. Everyone does something wrong sometimes, and some more than others. There's a subset of people who are comfortable doing what they know is wrong and would oppose as wrong if others did it, as long as they see a personal advantage in the wrong act. In terms that start to become political, that means that some of us will exercise coercion on people if we see a likely advantage to it. So, in a society of people minding their own business, engaging in voluntary relations of all kinds, some will join together to violate the rights of others for their own gain. They are, after all, acting in their own self-interest: if they feel no particular moral discomfort in gaining by taking your property by force, why wouldn't they turn marauder? Joined together, they have little to fear from the opposition of one person alone.
An anarcho-capitalist will undoubtedly point out that people can voluntarily join together to combat such a threat to their rights, and that they're acting morally to do so. I agree, anarcho-capitalist friend! I agree further that it is necessary that they will do so; if they do not, cooperating attackers can operate with impunity. Once these marauders come on the scene, people of initiative will begin to patrol together, watching for people intent on robbery, murder, and destructive mayhem: the porcupine will grow quills. It's exhausting, though, and a bit impractical, for the butcher, the baker, and the candlestick-maker to patrol the neighborhood every night, and they're probably not the best at it anyway. Better to butcher, bake, and candlestick-make, and use the proceeds to hire tougher people to do the patrolling. Practicality dictates that they will formalize that agreement, compensating a standing force for the service of defending their rights. Hooray for the specialization that capitalism enables! This is all fine, voluntary, completely moral.
Imagine the situation at that point: Some members of a community devote a portion of their substance to supporting a force to defend their rights. Imagine that members of this force see a person being mugged at gunpoint on the street. They rush to their aid, as soon as the person being mugged shows their Tom Woods Rights Protection Action Force Supporting Members Club card --- visit YourFriendsAreWrongAboutRights.com for the free e-book!
No, obviously they don't, because they're reasonable and compassionate people, and recognize the urgency of the situation. They can not delay without irreversible harm being done, and if the person is a subscriber, they have an obligation to provide the service they contracted to perform. They help the person being mugged, and send them a bill for services rendered to a non-member, if that turns out to be the case.
No, they won't do that either; the person being mugged didn't request their services, and may well have preferred to handle the situation themselves. There was no voluntary transaction to form a moral basis for that bill. Still, if they go around helping everyone whether they've paid up or not, pretty soon nobody will bother to pay, which is hardly sustainable.
The force is put in the position of either acting without reason, without compassion, without moral justification, or without pay. (That last bit hurts most.)
The most likely solution, if they are moral enough not to turn marauder themselves, is to declare, "We are the rights-protection force for the community, and those who wish to live here will be required to support us. Remember: there are people who will steal what you have, and for them it is pure profit, while we provide a valuable service for your money." Is such a suggestion moral? Not entirely, if only voluntary interactions are moral. Nonetheless, some people will accept without question. Wiser members of the community are unlikely to do so without some say in what rights the force will protect, and what sort of force they will use. They will insist that such a force be subordinated to the people by a set of rules determined by the community at large, and that the community select some of its most respected members to be consulted on the application of those rules when urgency allows. Then, and only then, will they agree to allow and support such a force, at a rate agreed on by the community. If enough do, then the holdouts will be irrelevant to the imposition of the program, and it will go through anyway; most will at least provisionally cooperate so they can help limit a system that has admitted benefits. If it becomes intolerable, they will raise a force of their neighbors and dispose of their oppressive "protectors".
This is a best-case scenario; in many cases the force that becomes predominant will be more oppressive than what I describe. It is at least a local minimum of oppression, on the one side of which you have no organized opposition to marauding oppressors, and on the other side of which the protectors become oppressive. But in this best-case scenario, government has spontaneously arisen: you have a body of laws determined by some democratic system, applied by judges, enforced by police, and funded by taxation. I believe this is the minimum of infringement on the absolute rights of the people that can have stable existence.