Controller SessionStateBehavior is ReadOnly and I can update Session Variable

I expect that if controller has attribute SessionStateBehavior.ReadOnly then I can't change session variables inside this controller but I can change values.

I try this code

 [SessionState(System.Web.SessionState.SessionStateBehavior.ReadOnly)]
    public class GLobalController : Controller
    {
      public  ActionResult Index()
        {
            Session["xxx"] = DateTime.Now.ToString();
            return View();
        }