Starting with JDK 1.4, using the middle mouse button to paste text into a JTextArea ignores whether or not the text is editable.  ie you can paste into a test area that has isEditable() == false.

I can't point my finger at the exact fault/change in the source, but I suspect the change is due to the changes involving invokeAction and TransferHandler.getPasteAction.  It would seem that you can now get at the paste action via the action map; unfortunately, this bypasses the isEditable check in JTextComponent.paste().