1. Amazon is --- probably, and for the next 18 months or so --- going to have the most popular Android tablet on the market.
2. Amazon does not provide mainstream users with access to Google's Android Market.
3. Android developers are going to be compelled to deliver software to Amazon users, because Amazon is going to supply a plurality of the entire addressable market for Android apps.
4. To supply Amazon users with Android apps, developers have to use the Amazon market.
5. Not only will developers need to use the Amazon market to address Amazon customers, but off-brand Android users can (and may) also end up relying on Amazon's market.
Google's Android Market is structurally disadvantaged. And, simply having to sell through Amazon is already a hardship for developers; Amazon has policies that developers are going to have to comply with, even though those policies aren't inherent to the Android ecosystem.
Saying that Google doesn't have to care about native apps, that the web is where it's at, that they're "skating to where the puck is going to be" --- that's all besides the point. The concern is specifically about the Android app ecosystem. If you don't think it matters that Amazon is redefining that ecosystem, well, OK, but that doesn't make it not a concern.