In C++, why struct is in fact class?

Question

The other topic and responses there made me ask this question:

Why does C++ allow struct to behave just like class? At one hand, C++ made

Answer 1

It allows existing structs to be fitted in with C++ code in a more natural way. For instance, you can add member functions to a struct and inherit from a struct, which wouldn't be possible if structs and classes inhabited different universes.

Stroustrup's original intent was to avoid a fracturing of the community between the traditional C-style "struct" camp, and the OO "class" crowd. He also cited the benefits of having just one concept instead of two.