Not sure what the legal system there is. According to the article, the health insurance sets what you may do when on sick leave. Their health insurance apparently even limited the times when they could leave their house. Incredible, this is like jail!
Whilst you will regret working, say a push cart, when on sick leave with a bad back (for your employer will use the case to lay you off), all the rest is complete and utter BS.
E.g. I had a sore back few weeks ago and went to the doctor, got a few days sick leave and was told to go swimming and see the physiotherapist. No times given, nor would they be restricted by the insurance or anybody else.
I´m German and pay into the German health system since 1983.