Actually, they don't. Neither the MapReduce nor nearly all items on Stroustrup's list fall under conventional definition of the "system programming". It is an established term that does not mean programming of a system. It is rather a synonym of the "OS development".
I include it to mean support software that sits between the OS and applications, like a runtime system or a memory allocator. Much of that code (and MapReduce) qualify.
A high profile, recent example is Google's MapReduce: http://labs.google.com/papers/mapreduce.html.