Why doesn't the USPS cancel "nonprofit" stamps?

Every time I receive something in the mail from a non-profit organization, it comes with an uncanceled “USA NONPROFIT ORG.” stamp on it. That stamp, unlike the regular stamps, is never cancelled by the United States Postal Service. Why is that? What is keeping (besides, like, the law) from cutting it off and putting it on letters I have to send? Does the USPS have a way of detecting whether an organization is legitimately non-profit or not?

Most likely because the USPS is requiring the non profit to deliver the mail to the local post office pre-sorted. I don’t think it actually ever passes through an annex or sort facility for the postage to be canceled. I suspect the auto sort machines they use will kick out any letter with one of these stamps if one attempted to use them for normal first class delivery.

Having worked for the USPS for over 15 years in the main sort center in west-central Florida, I can say for sure, if you are not mailing out hundreds (or more) of identical mail pieces, pre-sorted to some level (city, carrier, etc.) they will know you have not qualified for the Non-Profit Presort rate. Also, the Non-Profit (or other Bulk Business (junk)) mailer will have several levels of sorting, requiring different prices per mailpiece. There will usually be a USPS employee onsite at the sort facility to ensure the business’ compliance with the presort rate the mailer is claiming.

Just because a stamp is not “cancelled” does not mean it is still usable as postage, even a first-class stamp. If they determine a stamp has been re-used, they will return the mailpiece “insufficient postage.” If they determine a pattern of willful misuse, they can, and have, charged those involved with mail fraud.

There’s also no reason to reuse the stamps: even if a nonprofit managed to gather them up and reuse them, they pay for the mailing by the piece when they bring the bulk items to the post office.