I've been pondering this because on the face of it it sounds stupid and counter-productive. I want to think that the decision makers aren't complete baboons and at least thought they were doing something good. What i think they might be thinking is this:
In germany workers have EXTREMELY strong sick protections and take every chance to get paid-sick-leave as mandated by law they can.
And the main way this happens is the flu, which knocks a person out of work for two weeks over here. And this is unavoidable, people get the flu, people get sick. It happens. Especially in group environments.
But if people are individually on the hook for their flu stuff, have it affect their bonuses, then they have an incentive to show up to work, not get their days counted, but get everyone else sick. Out of those there will be a number of people who do take the sick days.
However if you average the bonus sickday connection out over a group, then suddenly the sick people have an incentive to stay home. If they don't infect coworkers, they don't raise the sick days of the group en masse, thus affect their own bonuses, and everyone else's less than if they went to work sick and got everyone else sick too.
At least that's what i think the managers are thinking.
Whether that works out like this here, i can't even imagine.