With the risk of starting another flame war. There is no law without some authority that has the power to enforce the law. And once someone has the power to enforce the law, he has the power to create and change law. You really cant keep law enforcement and law creation separate. Whoever has the power to enforce the law, is the state. Its a top down thing, because the law must be based on the treat of violence from someone everyone fears. Law cant be a bottom up thing, the law (or the payment of law services) cant be voluntary, because there nature of law itself is based on violence and authority.
The utopia is thinking that we can employ someone to protects our rights, without in the process giving away our power to ensure that this party does not abuse his power. The problem is there are no checks and balances that keeps the person/company who is employed to provide law enforcement, to staying a servant and not deciding he is a ruler.
Imagining that the state magically dissappeared overnight and a number of private companies where allowed to take its place. So the theory might go that instead of having a state charging for its protections service threw forced taxation, that the companies get payed for its protection services threw voluntarily payment. Well this is naive, because what is to keep these private companies from morfing into the state (or states). What is to keep them nice, and wait for customers voluntarily to pay them for their service, what is to keep them from force customers to pay it for its services (at gun point and threats of violence). What is to keep these companies from turning into mafias, gangs, militias and states? Nothing, because the guys with the guns make the rules. Waiving a contract or the law in their face, and telling them.. "but.. but you promised!" or "you work for me, remember?". Will get you nowhere. The only reason people and companies respects the law and contracts is because there is some bigger stronger authority than themselves with the power to violently punish rule breakers. In the market anarchist utopia there is no such authority. Law creation and law enforcement comes from the top down, you cant make it into a bottom up system, however nice that would be. You cant make the law (and payment for it) voluntary instead of mandatory, however nice that sounds.
Or imagine this, you and a group of people are on a deserted island. Everyone is given a job, and everyone is paid the same aount of bananas. One of the peoples job is to keep the law, asn he is given the only gun in order to do so. Its abit naive to think that just because you pay him bananas that he works for you. Him having a monopoly on violence will decide the rest of you work for him and he will take as many bananas as he likes. Well what if there was two guns on the island, and these where given to two different people, both of who are supposed to keep the law. Well one of them might just kill the other, and get all the bananas he wants. Or instead of killing eachother they might make a deal to splitting the island and its people between echother. They would draw a line in the sand and agree that the people and the bananas on the other side belongs to the other person with the gun. There now would be two govts and two countries on the island. The point is that without a state, there would be nothing that kept the private law enforcers from becoming the state.
Cheers