Those regional currencies have popped up several times, the Chiemgauer is just the only(?) long-living one.
Official stance IIRC is: illegal, but so small and irrelevant by volume that it's best to ignore it and let people have a little fun and community engagement.
It isn't. Only the Bundesbank / ECB is allowed to provide a currency in for Germany.
https://de.wikipedia.org/wiki/Währungsmonopol
The Chiemgauer is a regional voucher system - more comparable to regional loyalty programs or frequent-flyer programs.