Most libertarians reject the utilitarianism as a moral philosophy because it would seem to grant people the right to initiate force upon one another (via the state) so long as the cause is thought to promote happiness. The paradox is that, while a free society does not aim at the maximization of happiness, only in a free society is the maximization of happiness possible.
Some things are obviously detrimental to overall happiness. If people in a free society choose, for example, to smoke, it is obvious that this is less utilitarian than to feed the world’s poor with the cost of the carton of cigarettes – especially if the smoker has got to the point (which many do) where they no longer even enjoy smoking but just do it because they are addicted to it. If people in a free society pursue diets that lead to chronic illness later, all we can say is: “I would choose differently.”
Nonetheless, whatever one might do to regulate or stop any such “non-utilitarian” decisions would no doubt result in more misery over the long term. Take for example Prohibition in America which greatly expanded the power of the mafia. Or consider the War on Drugs which – in addition to being waged at incredible expense – has separated fathers from children and left people to rot for the crime of smoking a plant, while bringing cartels of gangsters to South America. Regulating non-utilitarian behaviors always bears a high price tag to the taxpayer which would no doubt create more happiness if it was allocated by the consumer to buying those things that they at least believe will maximize their pleasure.
In addition, when it comes to freeing the world’s poor of poverty, the massive economic growth created by the conditions of freedom far outpace the money thought to be “wasted” by truly consistent utilitarians on the caprices of the consumer. It may seem, on the face of it, “unutilitarian” that we “allow” the poor of the world to live on less than $1.90 a day while billionaires heat their outdoor swimming pools. Can’t we just tax the rich and send it to Africa? We can leave aside the point that when this has been tried the funds have invariably been wasted by dictators and central planners. Those who actually take the time to understand the market process, and how poverty has actually been eliminated in all those nations where it has, can look to the horizon and understand that it is in fact the assets of the wealthy which are destroying poverty and maximizing utility. All those billions are invested in the factories, machines and technological research which are pulling those nations which have moved from command economies to market economies out of poverty as we speak. Redistribute the money to the poor and they’ll soon be poor again, all the while destroying their employment prospects through lost wealth that could have been invested in wealth-creating industries and technology.
Allow the market to allocate resources to their most profitable ends (according to supply and demand) and companies will rush to world’s poorest nations to take advantage of cheap labor and develop sustainable infrastructure which will bring them out of poverty for good. In Bangladesh, the number of extremely poor fell from 44 to 26 million, and poverty in Cambodia has been cut in half. We see this trend all across the world. To the extent developing countries free their markets poverty falls – while those countries that hold onto autocratic control of the economy remain impoverished.
The paradox of libertarian opposition to utilitarianism is that when we resist the temptation to regulate people into pursuing happiness for a quick fix, over the long term, the market maximizes utility.
via Mises Institute
 Most notable exceptions being David Friedman, and then Mises, Hazlitt and Hume who were “rule” utilitarians, which is slightly different.