Scanning mail for sorting purposes is well and good and the USPS has done so for a long time. The issue I have is record retention. For mail that isn't tracked, there is no purpose for retaining routing records and certainly no postal purpose for retaining images of the mail.
What about stolen mail? That is a postal purpose to know what comes in, actually gets delivered.
Sure, that isn't the main goal of this program, but, it still has good uses, and as with everything else, it can be abused.
However, like I mentioned above, they will scan everything anyway, and signing up doesn't add more scrutiny to your mail, it just informs you what is coming.
Main use would be for law enforcement agencies to find patterns of drugs / threats / and other things that concerns them, but, they still need a warrant to open anything.