Clipboard operations are restricted in these browsers and when the paste tool is used nothing is happening. An alert should show up just like the Copy and Cut tools are designed.
A possible workaround is using the same functionality as the Cut command by incorporating the following JavaScript line of code
<telerik:RadEditor ID="RadEditor1" runat="server">
</telerik:RadEditor>
<script type="text/javascript">
Telerik.Web.UI.Editor.CommandList["Paste"] = Telerik.Web.UI.Editor.CommandList["Cut"];
</script>