I don't know what's going on in Germany now. Thirty years ago, according to a German I knew, you had to pay for the power you didn't use if you generated your own.
But bringing up spinning reserve and dispatch issues is helpful. The research paper from LBL that spawned the Cmputer world article has the title "Financial Impacts of Net-Metered PV on Utilities and Ratepayers: A Scoping Study of Two Prototypical U.S. Utilities". The key is "Net-Metered". In the two locations that I know about, customers are paid at least the same price per kilowatt-hour for energy that they provide as they pay for energy that they consume regardless of time. Since the cost of energy in electrical power girds changes over time, such pricing is not appropriate. Also as Luckyo points out, there is a cost for providing "spinning reserve" which is the capability of providing additional power on a fraction of a second's notice. Such reserve is necessary in order to have reliable power delivered to variable loads.