When RadEditor's ContentAreaMode is Div the entire page is printed instead of just the content. Possible solution is changing the mode to Iframe as it is by default.
You can use the solution provided in the following KB article:
Print Content when using Div ContentAreaMode, e.g.
<telerik:RadEditor RenderMode="Lightweight" ID="RE1" runat="server" ContentAreaMode="Div">
<Content>
<h1>Title</h1>
<p>Some content.</p>
</Content>
<Tools>
<telerik:EditorToolGroup>
<telerik:EditorTool Name="Print" />
</telerik:EditorToolGroup>
</Tools>
</telerik:RadEditor>
<script>
Telerik.Web.UI.Editor.CommandList["Print"] = function (commandName, editor, args) {
var prtContent = editor.get_contentArea();
var WinPrint = window.open('', '', 'left=0,top=0,width=800,height=900,toolbar=0,scrollbars=0,status=0');
WinPrint.document.write(prtContent.innerHTML);
WinPrint.document.close();
WinPrint.focus();
WinPrint.print();
setTimeout(function () {
WinPrint.close();
}, 1000);
};
</script>