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 document with redaction marks. If the text is printed using a laser printer or a type writer, and the marker used for redaction uses some other kind of ink - let's say one that doesn't dissolve the text's ink or toner in any way - then you can in principle distinguish between the two and thus recover visibility of the text.)
File formats are complicated. The only reliable way to redact is to reduce that complication to one which humans can manage. This is even true for software that is written by humans.
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.
(A better analogy might be the original physical document with redaction marks. If the text is printed using a laser printer or a type writer, and the marker used for redaction uses some other kind of ink - let's say one that doesn't dissolve the text's ink or toner in any way - then you can in principle distinguish between the two and thus recover visibility of the text.)