If I'm understanding the OP correctly, the C standard says so, i.e. the semantics of memcpy are defined by the standard and the standard says that it's UB to pass NULL.
Unlike all the more complicated languages the "freestanding" mode C doesn't even have a memcpy feature, so it may not define how one works - maybe you've decided to use the name "memcpy" for your function which generates a memorandum about large South American rodents, and "memo_capybara" was too much typing.
In something like C++ or Rust, even their bare metal "What do you mean Operating System?" modes quietly require memcpy and so on because we're not savages, clearly somebody should provide a way to copy bytes of memory, Rust is so civilised that even on bare metal (in Rust's "core" library) you get a working sort_unstable() for your arbitrary slice types!
