hi! In the event that you don't get answers here, you might try x-posting to /r/soccer and/or /r/bundesliga

Since there is a comparative European dimension to your question, I hope you don't mind if I mention some things from a Swedish perspective where there are similar rules in place by the Football Association (i.e. not mandated by law). In Sweden there is the 51% rule, saying a football club must own 51% of its shares and still be member-run if it becomes a company. Clubs being allowed by the Swedish football association to become corporations is somewhat recent and is mostly limited to a minority of clubs in the top flight. Historically, the relationship between money and sports used to be all the more controversial. It was argued that money would corrupt the sport. Professional Swedish fotballers such as the Gre-No-Li lineup playing in Italy were not allowed on the national team in the 1950 WC because they were not amateurs despite being among the very best Swedish players. This rule was dropped later in the 50s and the 1958 silver medal helped bring acceptance to the change.