I never said it can never be justified. I live in the EU and agree with most of our consumer protection laws. I'm just drawing an important distinction.
Not participating in a transaction is a personal issue, which doesn't need justification. Not letting others participate in it should need to be very well justified, at least in a State that is supposed to be democratic and just.
Personally, and in this particular case, I find the justification lacking.
That might be a moral problem for libertarians. But there are lots of (not only German) laws along these lines.