With all that has happened with the unions in Wisconsin, and now spreading to other states, some people are wondering what the libertarian position is on unions.
First, let's distinguish between what are referred to as private sector unions and public sector unions. Private sector unions are made up of union workers in the private sector (think General Motors, although this may be a bad example now that it has been bailed out by the government). Public sector unions are made up of union workers who work for the government. The battle in Wisconsin was over public sector unions.
A consistent and principled libertarian would actually have no problem with private sector unions if they were not supported by the government. Libertarians are often viewed as "anti-union", but it doesn't give the whole picture. The reason most libertarians are anti-union right now is because the unions, in general, push for government protection, in this case meaning government force.
The unions have been known to use violence themselves against other workers trying to replace them. But aside from this, even the so-called peaceful unions are advocating that the government use force on their behalf. Libertarians have no problem with a group of co-workers getting together and demanding better wages or better working conditions from their employer. If they are not obligated by any contracts, they can even go on strike. The problem is that the government uses force against the employers in not allowing them the option to fire the employees. Freedom of association is a two-way street.
Things get trickier with so-called public sector unions. An anarchist libertarian does not believe in any government, so there is no issue there. A panarchist libertarian does not really care what any other governments do, as long as they or any other individual is not subjected to any governments in which they don't want to participate. The issue could arise for a minarchist libertarian, but even there you are really only talking about judges and police officers. No government school teachers would exist in a minarchist society. If you only had to deal with police officers and judges in a minarchist society, the total spending would be so small that unions would not be that much of an issue.
But let's deal with the situation we are currently in. Although consistent libertarians are against having government schools, the reality is that we have them right now. So should the teachers be able to unionize and bargain collectively? It really is very hard for a libertarian to answer, but I would think that if the government is spending taxpayer money, it should find the best deal it can (with what is considered acceptable for quality). If a government is putting a contract out for bid, it should go to the company with the lowest bid that can meet the acceptable standards, so why can't the same thing be done for government school teachers? I know, you can hear the wailing that teachers are underpaid and under appreciated, etc.
The whole problem goes back to government schools. There are probably many teachers that are overpaid and many that are underpaid. We don't know because there is no free market. If teachers had to compete in the marketplace, the best teachers would be those who would tend to get rewarded the most.
In conclusion, libertarians believe in freedom of association. When it comes to so-called public sector unions, it is difficult for principled libertarians to take a position because they already oppose the state.