Why is std::ctype's virtual destructor protected?

Question

I was trying to write

std::ctype<char> c;

but I couldn't, because that class's is both protected (directly) and virtual (indirectly, through facet).

But I can do this just fine:

struct : std::ctype<char> { } c;

This made me wonder, what's the point of making std::ctype's destructor protected?

For example, is it trying to prevent a misuse or mistake? If so, what is that?