There is much to be said on both sides of this issue.
On the one hand, open shop agreements seem to be fundamentally fair. If a firm has a closed shop, workers are essentially being coerced into joining a particular union in order to be allowed to work at that firm. This is (arguably) fundamentally unfair because people should not be compelled to join certain groups in order to have a job.
On the other hand, we can argue that open shop agreements harm those who pay union dues. From this point of view, unions benefit all employees of a given firm. The pay and rights that the union bargains for help those who do not join the union. This means that people who do not join the union are free riders who are benefitting from the union dues that others pay. This is (arguably) fundamentally unfair because it is not right for some people to get benefits that they choose not to pay for.
Therefore, this is a thorny question. The best argument for this proposition is that it is not fair or right for people to be allowed to “free ride” and get benefits that they consciously choose not to pay for.