I know that Rothbard had already said this, but I want to add in my own particular take on opposition to anti-trust.
In my first economics textbook (obviously a neo-classical one), the concept of price elasticity was introduced via an example of a movie theater attempting to set a price for tickets. In this model, if $9 is charged per ticket, one person will buy it; if it costs $8, 2 people will purchase tickets, 3 for $7, and so on. There is no marginal cost for taking on another consumer (a questionable assumption, but one immaterial to the idea of the legitimacy of anti-trust), and the underlying assumption is that by withholding space from consumers the movie theater is causing harm to the general public.
In actual fact, by withholding seats, the theater owner is aiding the consuming public. Due to the heterogeneous nature of capital, there is a need for a medium of exchange and unit of account in order to calculate the allocation of scarce means, i.e. money. The means by which the theater owner determines the success of the arrangement his capital goods, such as his land, to make a movie theater as opposed to a variety of alternatives such as restaurants, bars, electronics stores, car dealerships, etc. is the amount of profit he could make from each, weighed against a variety of criteria such as expenses, interest, risk, etc. If the theater owner is forced to sell at such a price that he makes no profit whatsoever, there is no way for him to see what the most pressing need of consumers are that he can sate. However, if he is allowed to withhold his goods, he is able to see that, say, consumers will pay a great deal for movies, whereas they are uninterested in buying VCRs and he will get a very low return on what he has invested, indicating a misallocation. Were it not for profits, there would be no indicator for the theater owner as to whether his entrepeneurial decisions have proven wise, and he would be just as likely to start an establishment that served low-priority needs of a few as he would the high-priority needs of many since his opportunity for entrepeneurial profit has been taken away.
In the end, the elastic portion of potential theater attendees is helped by the owner's withholding, as it helps indicate to him that they want him to continue to sell his product. The inelastic portion is helped as well, as though they may be given lower prices in one particular product the elimination of indicators to determine the most advantageous arrangement of inputs would cause general misallocation of resources that would not satisfy their needs as effectively.