Why can struct change their own fields?

Question

Consider the Foo struct as follows:

struct Foo
{
  public float X;
  public float Y;

  public Foo(float x, float y)
  {
    this.X = x;
    this.Y = y;
  }
         


        

Answer 1

Found my answer at the very bottom of: http://www.albahari.com/valuevsreftypes.aspx

However, I still don't understand why the compiler allows mutable structs in that case.