Article 9 concerns freedom of coalition.

It stipulates:

1. Alle Deutschen haben das Recht, Vereine und Gesellschaften zu bilden.
2. Vereinigungen, deren Zwecke oder deren Tätigkeit den Strafgesetzen zuwiderlaufen oder die sich gegen die verfassungsmäßige Ordnung oder den Gedanken der Völkerverständigung richten, sind verboten.
3. Das Recht, zur Wahrung und Förderung der Arbeits- und Wirtschaftsbedingungen Vereinigungen zu bilden, ist für jedermann und alle Berufe gewährleistet. Abreden, die dieses Recht einschränken oder zu behindern suchen, sind nichtig, hierauf gerichtet Maßnahmen sind rechtswidrig.
   Maßnahmen nach den Artikeln 12 a, 35 Abs. 2 und 3, Artikel 87 a Abs. 4 und Artikel 91 dürfen sich nicht gegen Arbeitskämpfe richten, die zur Wahrung und Förderung der Arbeits- und Wirtschaftsbedingungen von Vereinigungen im Sinne des Satzes 1 geführt werden.

Let us dissect and translate this article by paragraph.

Paragraph 1 states:

All Germans have the right to found clubs and societies.

Nothing revolutionary.

Paragraph 2 states:

Associations whose purpose or whose activities run contrary to criminal law or who position themselves against the constitutional order or the thought of understanding among nations are forbidden.

This paragraph is an easy shortcut to getting rid of a good chunk of fascist bile. Invite an advocate for the law and order of Mussolini’s Neo-Roman empire? “Promoting a system that seeks to overthrow the constitutional order”. This is a powerful and underestimated paragraph. When there is no way to go after an individual, seeking to tarnish the image of their association is a lot easier.

Paragraph 3 states:

The right to form associations for the purpose of keeping and fostering the working and economical conditions is guaranteed for everyone and all professions. Agreements that seek to restrict or impede this right are void, measures aimed at this are illegal.
Measures in accordance with articles 12 a, 35 paragraph 2 and 3, Article 87 a paragraph 4 and Article 91 can not be directed against labour struggles which are conducted by associations for the purpose of keeping and fostering the labour and economical conditions in the spirit of paragraph 1.

The word that is implied but not explicitly stated is union. A contract that restricts workers’ rights to form unions is void, working to undermine that right is against the law. Reading this one strictly, there is basis to throw union busters into prison, even for the intent.

The second part, added in 1968, was introduced as a compromise to the strong protests against the German state’s resurgence of military might and out of fear for the dangers of unrestrained capitalism.

In conclusion, the labour union was recognized as a strong tool to introduce and foster democratic thinking not only every four years at the urns, but also every day at the workplace. They still form the backbone of every democratic socialist movement, and to ignore their voice even in the face of declining membership is to throw away elections.

The same article that sought to protect unions above a lot of other things also placed strong tools into the hands of union busters who would argue, sometimes rightfully, that the all too close friendship and mutual understanding between labour unions and the eastern bloc sought to introduce a socialist order to the young German state.
While socialist ideas do not have to endorse communism, those days, who could tell the difference?