The same issue was reported a while back in our support system. Here is the information we have on it:
The IE7 browser focuses on increased security and accessing the clipboard from client script is not allowed anymore (as was the case in IE6). Our editor implements a special workaround so you should not see this warning when pasting with the keyboard shortcuts, however trying to paste using a tool on the toolbar is guaranteed to issue a warning.
Why is that so?
In the first case (shortcut) the browser correctly assumes that it is the user intention to perform a paste.
In the second case the Paste command is activated through a script (which can potentially be malicious).
To remove any warnings, you should add your site to the list of trusted sites.
There is no workaround around the issue, as it is not a bug, but built-in browser behavior.
the Telerik team
, the state of the art learning tool for Telerik products.