Why does const imply internal linkage in C++, when it doesn't in C?

I believe you mean

Why does const imply internal linkage in C++

It's true that if you declare a const object at namespace scope, then it has internal linkage.

Appendix C (C++11, C.1.2) gives the rationale

Change: A name of file scope that is explicitly declared const, and not explicitly declared extern, has internal linkage, while in C it would have external linkage

Rationale: Because const objects can be used as compile-time values in C++, this feature urges programmers to provide explicit initializer values for each const. This feature allows the user to put const objects in header files that are included in many compilation units.

As litb said, const objects have internal linkage in C++. This is because they are intended to be used like this:

// a.cpp
const int BUFSIZE = 100;
char abuf[BUFSIZE];

// b.cpp
const int BUFSIZE = 256
int bbuf[BUFSIZE];

In C & C++ the term static has multiple meanings (it can govern linkage and storage) You'll have to read Stroustrup's D&E to appreciate his rationale - but when you declare a variable to be const at namespace scope it automatically has internal linkage - whereas in C you have to declare it static to force it to have internal linkage.

Of course in C++, the use of static to control linkage has been deprecated, anonymous namespaces can be used to simulate internal linkage in C++.

const variables in C++ were supposed to replace preprocessor constants - and since preprocessor constants are only visible in files that define them, similarly, const automatically makes the variable visible only in the file that defines it.