Is It True That It's Illegal To Mandate Green OA Self-Archiving in Germany?

We have heard it repeatedly claimed, without evidence or argument, that in Germany it would be illegal to mandate that authors self-archive their final drafts of peer-reviewed articles because it would be a "violation of academic freedom." I have never believed this claim, and do not even think it is coherent. A Green OA self-archiving mandate leaves authors free to publish whatever they wish and to publish it in whatever journal they wish. It merely requires them to deposit the final, accepted draft in an institutional repository.