See the link to the English translation of the Product Safety Act that I posted in another comment. Article 4 is relevant.
Just because its optional doesn't make it less of an secret law. It simply mean that the secret law can be ignored, and where those with privileged access can gain an market advantage.
Either the law makers is in fault here for making part of the law secret, or DIN should loose their copyright to the standard. You can not have laws with open and secret parts, or you creates a law enforced unfair market.
If DIN has proprietary ownership of the optional part of the law, DIN can then at their choice exclude companies at will. After all, proprietary ownership means the right to not sell a license to companies of your choice. This is clearly incompatible with a fair and free market.
In my view, the law should be rewritten so the standard is not included there. If the market want to make the standard an industry practice, or lawyers want to view it as the defined compliance to the law, then thats fine. That would mean that a competing standard writing body (or lawyer firm) could come up with a new standard, and compete in an fair and free market with the first one. As it is now, no such competition can happen since the standard is written into the actually law as a way to optional be in compliance. Thats an clear unfair advantage, and facilitates antitrust.