ASP.NET Setting parameter for SqlDataSource programatically does not work for postback

I want to set a parameter for a SqlDataSource programmatically as described in Step 5 at http://www.asp.net/data-access/tutorials/using-parameterized-queries-with-the-sqldatasource-vb . Also a GridView is bound to the sqlDataSource. My markup is:

<asp:SqlDataSource ID="mySqlDataSource" runat="server" 
    ConnectionString="<%$ ConnectionStrings:ConnectionStringHTL %>" 
    SelectCommand="SELECT [subscription_con] FROM [HTL_CONSUME_CON] WHERE ([subscription_con] = @subscription_con)">
    <SelectParameters>
        <asp:Parameter Name="subscription_con" Type="Int32" />
    </SelectParameters>
</asp:SqlDataSource>
<asp:GridView ID="myGridView" runat="server" AllowPaging="True" 
    AllowSorting="True" DataSourceID="mySqlDataSource">
</asp:GridView>

In the codebehind, I have:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load    
    mySqlDataSource.SelectParameters("subscription_con").DefaultValue = calcResult()
End Sub

The return value from calcResult() is different for each postback. The postback occurs when the user clicks a Button on the form that has UseSubmitBehavior=True.

I've use the debugger to step through the code behind and I see it executed for each page load and I see the expected values returned from clacResult().

However the bound DataGrid is never updated on the postbacks, it only updates on the first page load.

If I change the SqlDataSource parameter to have a control as the source, then it works on postbacks. In otherwords, I changed the markup to use:

<asp:ControlParameter ControlID="myTextBox" Name="subscription_con" PropertyName="Text" Type="Int32" />

and I changed the code behind to be:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load    
   myTextBox.Text = calcResult()
End Sub

With the TextBox control as the source of the SqlDataSource parameter, update of the GridView works for the original page load and all postbacks. However, I really have no need for the TextBox and would prefer not to use it.

What am I missing about how to set a parameter programmatically for an SqlDataSource? Why doesn't the bound GridView get updated on postbacks when setting the SqlDataSource parameter programmatically when there is no control source?