I'm no expert on the subject, but my best guess is that the Stamp Yearbook was given as a premium to instill good customers relations and in order to avoid any problems either for the USPS or the company you work for, or their employees and/or even tax implications on a so-called "gift", they stamped it "Not For Resale" so it doesn't wind up as an item of value on the secondary market and have someone pocket the money that could be received for it. Not that it's worth all that much, but with
ebay these days, one never knows what someone would pay for it.
The other possibility is that the USPS representative was given "seconds" of the Stamp Yearbook that may have had minor defects that would make them unsaleable at the retail level, yet still in fine enough condition that they made for great giveaways.