This is a massive unauthorized LOAN.
It strikes me that way, of course, but I am conservative in my financial dealings. People are calling for the USPS to operate like a business. Well, a private business is not required to put gift-certificate proceeds into escrow.
In fact, by suggesting people buy postage in large quantities in advance, it increases the chance that some will be lost. Every lost or destroyed stamp is revenue without obligation*, or nearly pure profit, if we figure a replacement stamp will be needed. So "every forever stamp sold today represents a stamp not sold in the future" is not technically correct and I'd be willing to bet that difference is significant.
This was a lame-duck thing from the Republican Congress of 2006 after they were voted out of office, IIRC. It was signed by George W. Bush a little over four years ago.
And the costs of the stupid Sarbanes-Oxley Act are also dumped on the USPS now, just like for businesses. I know how much SOX has bled businesses I work with, so I can only imagine the costs to the USPS.
*I'm not sure if those terms are the appropriate jargon, but I'm not trying to use them in a technical sense--I'm not an accountant, etc..