Die Allgemeinheit habe ein berechtigtes Interesse daran, religiös oder weltanschaulich motivierten Parallelgesellschaften entgegen zu wirken, argumentierte das Bundesverfassungsgericht.
Sounds very reasonable to me. If you are born into any kind of fundamentalist family, like a religous sect or ultrareligous practioners for that matter, or some kind of racist hate group, you have no chance but to be indoctrinated.
Despite all criticsm concerning public schools, justified or not, the act of homeschooling can prevent innocent children from getting into touch with a pluralistic society.
Whether or not homeschooling should be allowed if certain conditions are met that prevent children to be isolated and totally dependend on their parents or legal overloads is a totally different matter, i wouldn't want to stand against homeschooling per se.
The state does have a right to prevent physical abuse of children by their parents, why shouldnt it be allowed to prevent mental abuse as well?