Earlier quoted context omitted.
They're likely viewing the electronic documents by analogy to photocopies with blacked out sections where there is nothing to distinguish the text from the redacting marks and nothing you can project out. They don't know the structure of the file format and how information in it is encoded or rendered, or even that there is a distinction between encoding and rendering. (A better analogy might be the original physical…
To complicated, the people doing the redacting pasted digital stickers ontop of the text, people are just removing the stickers.
Plain text and flat images are my preferred formats for things which must be redacted. Images require a slight bit of special care, as the example in the underhanded C contest highlights, but it's possible to enforce visible redaction and transcription steps that destroy hidden information.