They DID inform the public. I've been seeing the signs and advertisements for the free Windows 7 upgrade program for months now.
But just like any program, it has to start sometime... if they had announced the release a month earlier so you qualified, somebody who purhcased their computer a month before you did would be ticked off because THEY didn't get the announcement. It's almost like Microsoft would have to retroactively give everyone a fee upgrade, back to the dawn of computers, so someone doesn't get slighted. Just like any sale, it has to have a starting point. You were just on the wrong side of the announcement.
Even with that, it is incorrect to say you now have to pay full price for Windows 7. Your computer is still eligible to install the Windows 7 Upgrade... you just won't get the upgrade for free. Still, the price for the upgrade version is considerably less than the full retail version. Heck, you can buy a Windows 7 Home Premium 3-pack for $150... that's $50 per license and you'll be able to upgrade 3 computers.