Because the app isn't normally free and they have to "ring them up" to keep track of total sales. The developer gets their normal cut even though Amazon is giving the app away for free.
Got the majhong app yesterday and love it. Perfect for a tablet.
I had to go through some hoops to get the App Store though. When I went to the Amazon web site, I got the message the Amazon App Store was not available in my country. I'm in the US, but I guess this is similar to the issues people see on the Android Marketplace. Anyway, I went through my PC and had them send the download link to my email and downloaded on my a500 from that.