I'm intrigued: What part of German labour laws make it illegal to not tell you why your employer does not give you a voluntary payment (e.g.: A bonus)?
As the other commenter wrote: bonuses are variable compensation that is part of the negotiated contract/wage, so not at all 100% voluntary.
So the employer can refuse to pay the bonus, but they have to give a valid reason. There are two levels to this: first, basic employment law, which places very hard limits on what you can do. Second, many companies (like this one) have agreements between the works council and the company, agreements that regulate these and other matters and become part of the employment contract, and are therefore also legally binding and enforceable.
In all cases, the employer has to have valid, provable performance-related reasons, these have to be communicated to the employee in a timely fashion and the employee given a chance to improve their performance.
In my case, management was so clueless that they knew none of this, they thought that bonuses were gifts they can shower on people they favour on a whim, just like a king to his subjects.
It was a bit of a rude awakening. :-)
They then tried to create a "case" retroactively, but apart from the fact that there was no case, it also wouldn't have mattered at that time.
