That is exactly what they're doing, and here's why:
Germany has a minimum wage. As long as they pay above that, they can mess with bonuses above that as much as they want.
But workers have an additional protection:
If they get sick, and a doctor determines they're unfit to work (which they happily do for anything that isn't straight-up faking it), the health insurance will start paying half their wage, and the company has to continue paying the other half. The company also cannot fire the worker (unless the worker indicates unwillingness to get better).
A relative of mine got cancer. The company he was with ended up paying his wages for over half a year, because they had no legal way of finding out what was wrong with him, and he indicated he intended to get back to work asap, despite knowing he would never again be able to.
So for german companies workers being sick can be a considerable money sink, especially if they are not determined to actually build loyalty with their workers. Those companies end up doing things like this. The german national rail company does similar things as well already.