Charlie, I'm not so sure this statement is correct. There are different states within the larger country of Germany . The same way the USA has states within it. And if you were to talk to any German md'rs, they would tell you that certain states don't have that restriction. Only a few states within the country of Germany have the dire sounding anti-md'ing verbage.
However, I have no doubt, that are md'rs in every part of German, who ... (gasp) are finding old stuff. On a coin by coin basis, I'm sure it's no big deal.
Also: I bet you that even here in the USA, if you found a goodie of that caliber in any a) city property b) county property c) state property, or d) federal property: THAT THE SAME STUFF WOULD APPLY. What I mean is, don't think for a moment that the USA is any different.
To test theory, do this: Walk in to your city or county offices. Hold up a gold bar, or rare coin worth $1,000,000. Ask the city attorney:
"Hi. I found this on city property with my metal detector. It's worth $1,000,000 and has great value of antiquity as well. Is it ok if I sell it on ebay for my own enrichment? Or does it belong to the city coffers, and to go into the city museum?" Which of the following 2 answers do you think they'd give you:
1) "you lucky bum! Sure, it's yours to keep. The city doesn't need the money anyhow. And we have no laws forbidding "removing", "collecting", "harvesting", etc.... So it's yours to keep!"
or
2) "No I'm sorry, but items on city property belong to the city. You'll have to give up that item to us."
If you think they'd give you answer #1, then I have a bridge in Brooklyn I wanna sell you