And you should know why the German regulations are a bad example of your case: the German political laws (5% minimum vote before election, etc) are there to exclude the possibility of another Nazi party emerging. The laws in Germany aren't there for any good constitutional reason, they are there because of a political reflex against certain forms of extremism. No, that doesn't mean they are a bad thing, but it doesn't have anything to do with the constitutional validity of enforcing democracy in political parties by legal means.I have three quick reactions to this argument. First, in a democracy the dominant political party will inevitably legislate in a way that will advantage them, but when there is a strong civil society and real contestation for power a dominant party will think twice before adopting legislation that would obviously rig the system.
Second, even where the rules favour the dominant party it is always better to have regulation that not having regulation at all because without any regulation the dominant party who can distribute patronage and has wide state powers, can easily go wayward. In any case a party like the ANC who experienced severe upheaval in the preparation of its election lists before the last local government election, may come to see the benefits of legislation that regulate the selection of candidates.
Lastly, Germany is not the only country who has adopted party laws. Other countries like Mexico has also done so and did so in reaction to a long history of corruption in the politics of that country. In a country with pure proportional representation a party law can help to stop the internal party corruption in the compilation of party election lists.