Is there any good reason why C++ namespaces are "closed" in D?

[Walter Bright]

On 7/29/2018 1:45 PM, Manu wrote:
> There's no way you'll get a bug report from someone complaining they
> can't multiply define symbols in the same scope. That's common sense.

But then you cannot interface with this C++ code:

     namespace ab { void foo(); }
     namespace cd { void foo(); }

Why would you find this acceptable?