The Amazon Appstore is one of the worst apps I've seen. It's slow, buggy, and as everyone has been mentioning, inexplicably logs out periodically. The only reason I keep it is just in case there's a really good Free App of the Day. Otherwise, if I see something decent there, I'll usually go to Google Play to get it.
I wonder if Amazon does this on purpose to encourage people to buy a Kindle Fire (where I assume the Appstore is optimized to run smoothly), rather than other Android tablets.