Other cartels are illegal in most first world countries. Karl Marx wrote that labor was a fundamentally different commodity than any other. He was wrong. Unfortunately the US court agreed with him around the turn of the 20th century, and unionism was rampant through the 20th century.
When unions use force to become monopolies, they are usually destructive. Unions in fact try quite hard to become monopolies - compulsory unionism and demarcation work quite effectively to ensure that a particular labor buyer can only buy expertise from one union.
Unionism though, is slowly dying in many countries. Globalization will largely weed it out because workers in the first world themselves realize that if they force up the price of production then they will simply lose their jobs. In a free market economy this would not be a problem, but the effect of the artificially inflated market price of labor has been unemployment, so the unionists themselves are worried about becoming involuntarily unemployed.
Employers in many first-world countries have no legal right to discriminate on the basis of the membership of a union or employee organization, but employees have the right to choose their employers on whatever basis they want. Employees have freedom of association, but employers do not.
People on the right of politics try to prevent collective bargaining, but collective bargaining is not the problem. Collective bargaining is a natural process which (in itself) doesn't involve initiating force. As long as employers are free to buy their labor from other parties, and the laborers are free to sell their labor to another employer, the free market will do its thing.
Forcing an individual or entity to only only deal with one group is another matter. Blockading (picketing) and stopping others from entering a site (or threatening them if they do), is initiation of force. Threatening an employee with physical harm if they join a union is initiation of force. Damaging an employer's equipment or site is initiation of force. Threatening a co-worker with violence if they don't join the union is initiation of force. They are morally unacceptable.
Perfectly acceptable however is an action which does not initiate force. Terminating (or suspending) doing business with someone (provided it does not violate a mutually agreed contract) is not initiating force. An employee may leave a job for whatever reason they choose (contractual violations notwithstanding). An employer may suspend an employee for whatever reason they like (contractual violations notwithstanding), including not liking what they are wearing. A union may go on strike for any reason they choose (contractual violations notwithstanding), including lack of chocolate biscuits in the tea-room.
As long as an employer has a choice to go to another union to deal with, then there is no problem with a particular union making employment demands and striking to attempt to get them. However, thuggery, blockades, pickets, criminal damage to sites or equipment and threats of violence against workers employees or family members are initiation of force and should be illegal.
There is currently a problem with the unions and the government policies. It seems that a number of sources have criticized the unions for working against the government where unemployment is concerned. There is a system whereby these critics can be silenced, and everybody can be confident that the unions are representing everybody's interest.
If the unions want to be represent all workers, then perhaps they should volunteer to also represent all of the people who want to work, but can't due to the unemployment problem. If the unions were forced to allow membership of all people who want to work in a certain area, and if the union membership fee was directly proportional to the income of the member, (so that membership is in financial reach of everybody, not just those who can afford it), it would have serious consequences for unions.
Everybody who was a member of a union, but who was sacked due to some industrial dispute, or laid off due to raising labor prices and so on, could still be represented by the union until they were able to secure further employment, young people who have never had a job could join a union, and know that the union would do its utmost to find them one.
In this way the unions could be involved in redeployment of staff, in finding the workers more jobs, and in working with the companies in securing jobs for them. While the unions may say that job creation is the responsibility of the government, it is clear that this is too big a job for the government alone. Unemployment is the single largest problem that this nation faces today. It is a problem which must be handled by the employers, the taxpayer, the public, the government and the unions.
If the unions answered to the unemployed, as well as the employed, then the unions would have some justification for claiming to be a democratic representative of the workers - all workers, not just those who have been lucky enough to keep their jobs in a recession.
Unions frequently work against the interests of their members. For instance, a union may insist on overtime or 'penalty' rates of double pay on public holidays, but an employer decides it's not worth the extra cost, and just won't trade on that day. A particular labor seller (employee) may not really care whether they work on a public holiday or not, and may be willing to work for time and half, or even just normal time. The union is stopping them from earning money.
Confrontation-based unions are pretty much a thing of the past. The only way that most unions will survive is by actually providing services for their members. For example, providing legal representation when there is a conflict with the employer, or training to make them more productive.