I can delete digital signature from a protected document.
I have signed some test pdf document with a digital signature.
As you can see on a picture the document is totally protected from any modification.
Now I go to a signature panel and rightclick my signature.
There is a Clear Signature option, which do what it says - deletes my signature and make document editable again.
I understand the concept of signature. It is not a method to protect your data, it only can guarantee that content was not altered.
But never the less:
Adobe states in its documentation that you need a private key to unsign the document. It is not only rational. Its about following your own documentation.
I know a lot of software that breaks adobe security easily. This does not mean Adobe should ignore lesser security issues.
Such "Clear Signature" options make it possible for a receiving side to accidentally remove the signature. So its even a usability flaw.