Encoding - C# Multiline Textbox Allowing Input Above UTF-8?

In my web.config I have

<globalization
fileEncoding="utf-8"
requestEncoding="utf-8"
responseEncoding="utf-8"
culture="en-US"
uiCulture="de-DE"
/>

In my page directive I have

ResponseEncoding="utf-8"

Yet for some reason an ASP TextBox with the TextMode="MultiLine" allows inputs of characters outside of UTF-8. When I paste the following line of text into an ASP TextBox that is not MultiLine

the non UTF-8 characters are replaced, but not when I use a MultiLine TextBox.

EDIT: To explain a little more the set up I am seeing this problem in, here are 4 text areas that can be put on an ASP page.

<asp:TextBox ID="txtTest1" runat="server"></asp:TextBox>
<asp:TextBox ID="txtTest2" runat="server" TextMode="MultiLine"></asp:TextBox>
<input id="Text1" runat="server" />
<textarea id="Textarea1" cols="100" rows="8" runat="server">