The content area of the Kendo Editor is an editable iframe which uses the Rich Text Editing engine of the used browser. The issue stems from a browser behavior which we cannot work around.
You could test the behavior with a sample div with contenteditable attribute
set to true
where the browser will block the pasting of image and text in the same way as the editor widget:
"width: 400px; height:400px; border: 1px black solid;"