Asp.net mvc ajax.BeginForm Redirecting

I dont think i quite get the Ajax functions in mvc, because i get this wierd problem.

I got the following code which makes my ajax call, it is placed in a partial view with a productList:

                <% using(Ajax.BeginForm("AddToBasket", "Basket", 
                            new { productID = item.Id }, 
                            new AjaxOptions { HttpMethod = "Post", UpdateTargetId = "Basket", OnSuccess = "productAdded(" + item.Id + ")" })) { %>
                    <input type="image" src="/Content/addToCart.png" />
                <% } %>

I have a <div id="Basket"></div> on my masterpage

And this method in BasketController, which returns a partial view that is found in Basket/BasketList.ascx:

        [HttpPost]
    public ActionResult AddToBasket(int productID)
    {
        // DO STUFF

        return PartialView("BasketList");
    }

When i am logged in using the default asp.net membership it all works fine, it updates the basket and it is all async, but when i am logged out and is clicking the addToCart, it redirects me to Basket/AddToBasket?productID=1, which is a partial view.

Does anyone know why this happens?

I have a similar problem with an ajax.actionlink

<%= Ajax.ActionLink("Gem", "SaveBasket", "Basket", new AjaxOptions { HttpMethod = "Post" })%>

where it says "The resource cannot be found." when it should fire, which is placed in the BasketController

[HttpPost]
public void SaveBasket()
{
    // DO STUFF
}