Don't know why either; I have a PO box in a little two bit PO that has it (not Farmingdale as this PO is both closer to my residence and more importantly right next to a great discount liquor store, outstanding Korean restaurant where I'm the only non-Asian eating there and an outstanding pizza place).
Anyway, my point was that I was not surprised that all mail is photographed as I see all mail going to my box including junk mail.
The USPS, uses a system called Mail Isolation Control and Tracking (MICT), and it operates in over 200 proceeding centers throughout the US, since 4th quarter of 2001; photographing every single letter, and package over 160BILLION pieces every year; and provides the pictures to any law enforcement agency upon request (doesn't need a warrant or even probable cause, just because they want it). Though they claim officially they only keep the images for a week to 30 days; this can easily be proven to be a lie, since LE by the time they are ready to make requests typically are well outside a 30 day window.
If you know any USPS mail delivery guys, ask them they can tell you that images are stored for much longer. Especially when their asked about a particular mailing weeks later and have to review the scans and pictures
How many criminal investigations are ready to go to the USPS to get scanned envelope pictures within 30 days. Some investigations, especially the big ones can take up to a year before they start get evidence from email providers (SCOTUS and multiple courts have already ruled that email isn't protected after 6 months, LE can request as receive all content older than 6 months stored in Yahoo!, Hotmail, Gmail, etc. . Without any warrants or probable cause).
Of course, this was preceded by the "mail covers" program, that the police used to use before 2001 (it's a 100 year old program). they could request that the USPS copy the information on the letter for weeks, and months to particular addresses, again all without a warrant or probable cause