Because that's what it means to compile a specific dialect of a specific programming language?

If you want a dialect where they aren't allowed to assume that you would have to make your own

Sadness. Tons of functions from the standard library are special cases by the compiler. The compiler can elide malloc calls if it can prove it doesn't need them, even though strictly speaking malloc has side effects by changing the heap state. Just not useful side effects.

memcpy will get transformed and inlined for small copies all the time.

The most common reason is to do optimizations such as replacing strlen("hello") with 5 or open-coding strlen (or, more commonly, memcpy or memcmp). If you're linking with a non-conformant strlen (or memcpy or whatever) the usual thing that happens is that you get standards-compliant behavior when the compiler optimizes away the call, but you get the non-conformant behavior you presumably wanted when the compiler compiles a call to your non-conformant function.

But the orthodox answer to such questions is that demons fly out of your nose.

> What happens if it isn't?

§6.4.2.1: "If the program defines a reserved identifier [...] the behavior is undefined."

What happens when you change random functions in your C compiler? The C standard library and compiler are not independent, both make up a C implementation, which behaviour is described by the C standard.