USPS regulation?

While in Laredo, Texas  recently, I took an envelope into the USPS main branch.

The mail was addressed to Canada with my Mexico return address on the front of the envelope.

The agent said she could not mail the envelope with a foreign return (Mexico) address.  I started to walk away but instead went to the service counter, crossed out my Mexico return address and put in the name of the hotel I was staying at in Laredo. The USPS agent accepted the envelope with this change.

Does anyone know if this really is a USPS regulation?
Do the local mail services in Ajijic-Chapala replace a "foreign" return address for mail they take to Laredo for remailing?

Whenever I send mail to the USA I use my Laredo, TX forwarding address, not my MX address.

No, they do not change the return address, but if using postage stamps with a Mexican return address and the mail cannot be delivered in the US, it will wind up in the trash in the US.
Always be sure your return address matches the country of whose postage stamps are on it, whether done by hand or machine, at the origin.  Mail hand carried or via a mail service that is posted in the  US must have a US return address, otherwise, if not delivered it will not be returned.