Presumably because *this means something special in C++, and is not a common thing to write in C.
I've written this bug in C++ (memset(this, 0, sizeof(*this))) - clobbering your vtable pointer is pretty annoying to debug. My horrible hacky fix was just to zero the data portion of the class: memset(&firstMember, 0, (char *)(&lastMember + 1) - (char *)&firstMember)