Private variables accessible from outside class

[Drobet]

I'm having a weird issue, where after defining my classes 
variables as private, they can still be modified and looked at 
from the outside. That leads to this code compiling with no 
issues.

import std.stdio;

class Vector3
{
     this(double _x = 0.0, double _y = 0.0, double _z = 0.0)
     {
	x = _x;
         y = _y;
         z = _z;
     }

     private:
         double x, y, z;
}

int main()
{
     Vector3 vec = new Vector3(5, 5, 5);
     vec.x = 10;
     writeln(vec.x);

     getchar();

     vec.destroy();
     return 0;
}

My question is if this is intended behavior, and if yes, why?