Why does the STORE button on Amazon Kindle go to the website instead of the in app store?

I have the Android Amazon Kindle app installed on both my Galaxy S phone and my Archos 10" tablet.

I keep the software updated so I'm sure the latest version is installed.

I am logged in with my account on both devices.

When I press the STORE button I go to the inprog Kindle store. But when I press the STORE button on the Archos it starts the web browser in stead.

Now, I have bought ebooks before from both devices and I am sure that previously, the Archos also displayed the inprog store. It's much easier to use the inprog store display than the full Amazon web site version.

Does anyone now what could have changed this behavior ? I looked for some setting but found none.