OK. So I called Amazon and here's what the rep said. What you get in the email is just the market app to allow you to download/install the free app. It's not the actual app. You can't download the free app without an Android device registered with Amazon. So all of the ones I downloaded will be charged when I install it if it is no longer free. Bummer.
On the up side, he said that he's seen the free apps re-listed sometimes. So they may come through again.
Carol