Why do third party apps not respect document permissions?
Some third party apps, such as Apple Preview, don't respect all the document permissions set. If you try and open a restricted document in PDFpen or Acrobat and attempt modify, you'll get warned about the restriction and prompted for the password. Unfortunately third-party apps such as Preview can ignores the restrictions. All PDFpen products will enforce the restrictions set by the password, but not all third-party apps support and/or respect them, so if you're using an app that doesn't respect them such as Apple Preview, you're able to work around the restriction.