Disabled radio button losing value after postback

I have two radio buttons that are disabled with javascript when the page loads. RadioButton1 is checked by default. When I click the button to do a postback, the RadioButton1 is no longer checked.

Anyone know why ?

Here's my code sample. The code behind is empty.

<asp:RadioButton ID="RadioButton1" runat="server"  GroupName="group" Checked="true"/>
<asp:RadioButton ID="RadioButton2" runat="server" GroupName="group" />
<asp:Button ID="Button1" runat="server" Text="Button"></asp:Button>
<script type="text/javascript">
    window.onload = function () {
        var RadioButton1 = document.getElementById('<%= RadioButton1.ClientID %>');
        var RadioButton2 = document.getElementById('<%= RadioButton2.ClientID %>');

        RadioButton1.disabled = true;
        RadioButton2.disabled = true;
    };
</script>