https://en.wikipedia.org/wiki/Right_to_work
Article 23.1 of the Universal Declaration of Human Rights states:[1]
(1) Everyone has the right to work, to free choice of employment, to just and favourable conditions of work and to protection against unemployment.
Article 20 states:
(1) Everyone has the right to freedom of peaceful assembly and association.
(2) No one may be compelled to belong to an association.
— Universal Declaration of Human Rights, United Nations General Assembly
The International Covenant on Economic, Social and Cultural Rights states in Part III, Article 6:[2]
(1) The States Parties to the present Covenant recognize the right to work, which includes the right of everyone to the opportunity to gain his living by work which he freely chooses or accepts, and will take appropriate steps to safeguard this right.
(2) The steps to be taken by a State Party to the present Covenant to achieve the full realization of this right shall include technical and vocational guidance and training programmes, policies and techniques to achieve steady economic, social and cultural development and full and productive employment under conditions safeguarding fundamental political and economic freedoms to the individual.
— International Covenant on Economic, Social and Cultural Rights, United Nations General Assembly
https://en.wikipedia.org/wiki/Open_shop
https://en.wikipedia.org/wiki/Open_shop
Aside from the human-rights argument, there exist cultural and religious arguments too. Some cultures shun conflict and promote cooperation. Labour-management conflict is just one form of conflict. The purpose of a labour union is to organize against management. With that, depending on a persons beliefs or upbringing, though that person might join a labour union if necessary, he will not feel comfortable in it and might even have difficulty understanding its internal culture, in which case the union will be of no use to him if he can't understand its purpose or disagrees with its confrontational ethos.