It's also different than Google, which makes money from search. In theory, Google wants to make money on services and content sales, but it's still all about search advertising right now.
A $99 Kindle tablet would be a very disruptive move. Amazon is a quality brand. People admire and trust it. It's not like Asus, or Acer, brands that aren't as strong in the U.S. They could sell a $99 Android tablet, but people would be wary.
A super cheap Amazon tablet is a threat to Microsoft, which is nowhere in tablets and Google, which is just starting to gain traction in tablets.
It's less of a threat to Apple because Apple is playing a different game. It makes a more expensive, but higher quality — from software and hardware perspective — product.
In the long run, like in five years, it will be a problem for Apple because the price of an Amazon tablet isn't going up. The software and hardware aren't going to get worse, they're only getting better.