Why is there a FedEx box at the U.S. Post Office?

They’re competitors, right? The USPS offers overnight and expedited delivery, similar to what FedEx offers. From the way USPS advertises its faster service options, you’d think they wouldn’t want to help FedEx take away some of their business.
But I’ve seen more than one post office, including the one right around the corner here, with a FedEx drop box right by the front door.
So why is that? Is there some sort of law that forces them to let FedEx put a box there, or am I wrong that they are competitors?
(I understand that FedEx and USPS aren’t exactly equal in the services they offer, one may be better than the other, and there are reasons to use one in certain circumstances. But still, they’re both trying to get my business when I want to send a box to Uncle Earl in Omaha by Thursday.)

FedEx is a subcontractor for USPS. They ship priority mail and such. One of the terms of the contract is the FedEx dropbox that you see at the Post Office.

http://wired-vig.wired.com/news/business/0,1367,38641,00.html
http://www.bizjournals.com/memphis/stories/2001/01/08/daily9.html

USPS, FedEx Reach Pact:

Ahhhh… It’s heartwarming to see them playing nicely together.

IME, I’ve had FedEx packages sent to a rural location where I worked. It was about 50 minutes drive past the nearest PO on dirt roads. The FedEx guy would always drop the package as the PO for use to get.

Part of the agreement is that the Post Office will deliver FedEx packages into remote areas where it would cost a bunch for FedEx to send out a delivery person, but the USPS still needs to go to for making deliveries.

(I work on USPS engineering projects now and this is from my engineering counterpart at the USPS. Upon preview, I don’t know how this fits in with CynicalGabe’s experience.)