I am not 100% sure but my understanding is that the issue in Germany is not national or local regulations it is the rules set by those that manage specific sites. Often lakes and quarries are owned, leased, and/or managed by dive clubs that set there own rules that must be followed if one is going to dive at their site. At some sites, particularly in Germany, one must dive with 2 1st stages. I have been told there are some sites that dictate one must have a redundant air supply as well (twin set, pony bottle, 2 side-mount cylinders, etc.). Here in Belgium there are sites that forbid solo diving even the diver is certified to do so, and there are sites whose managing club do not recognize any certifications from organizations other than LIFRAS and CMAS.
All this is moot if one heads north to the coast, but that is often a long drive for so-so diving.
When I first moved here I was skeptical about diving in lakes and quarries but there is something to be said for diving in freshwater, when you get home you just need to hang your gear up to dry., no need for soaking and such. If I am going to dive in salt water and incur the PITA aspect of cleaning my gear, the diving better be worth it. If not I would rather blow bubbles in the local fresh water sites and save the wear and tear on my gear.
-Z