Why do some libertarians hate unions? Megan McArdle says that liberals believe that libertarians hate unions because "they raise wages and improve working conditions for their workers at the expense of profits." Citing this article on tollbooths and the adoption of technology like the EZ-Pass, Megan says that the "the central problem with unions, to the extent that there is a problem, is not that they demand higher wages, but that they reflexively oppose productivity enhancing change." And she cites a couple of examples -- the dockworkers' strike and the Big Three autoworkers who sit around being paid to do nothing.
Megan is right that one problem with unions is that they tend to oppose productivity-enhancing innovation, and this is indeed a good reason to hate unions. But, from a libertarian perspective, the central problem with unions is that they depend on the coercive power of the state. Property rights and freedom of contract are fundamental rights. Under current law, if 51% of the workers at company X vote to join a union, the employer is legally required to contract with that union. (The Democrats want to change this to require compulsory contracting if 51% of the workers can be persuaded or intimidated into signing a card.) The employer cannot refuse to contract with that group, contract individually with the other 49%, offer payment to workers in exchange for an agreement not to join a union, or seek employees willing to give a better deal. Every private sector union contract in force today exists in part because a people with guns say "bargain with them, or else."
Under libertarian rules, workers would, of course, be free to form unions or join existing ones. They would be free to seek a favorable contract from their employer. By the same token, the employer would be allowed to bargain individually with people who chose not to join a union, to refuse to bargain with the union, or to offer contracts which forbade unionization. I am not a big fan of unions, but that is largely because, under current rules, they have the heavy hand of the state behind them.