It is just as easy to extend any of the scripting languages with C++ as it is with C. SWIG and other systems like XS have built-in support for C++. Even if that weren't true, you can just make your interface header files 'extern "C"' yet enjoy the superior power of C++ in your implementation files.
There is no reason to favor C over C++ when extending a scripting language.
There is no reason to favor C over C++ when extending a scripting language.