Laarbruch72, I can assure you that many, many stretches of Autobahn indeed remain unrestricted! Neither is there an 'advisory' 100 Kph limit. However, if you have an accident at above 130 kph, you have to prove that speed was not a factor.
The design requirements for Autobahns are vastly more demanding than the requirements for UK motorways. Radius of curves, subsoil stability, surface condition and the number of lanes lead to speed limits where engineering considerations require them rather than by the opinion of some nannying politician.
Angela Merkel has been very forthright about slapping down any weenie-greenies who want to introduce blanket speed limits - a refreshing, commonsense and adult way to treat the population.