There's a difference between storing every single tap and just storing the latest position for each book and device pair. And the Kindle doesn't update whispersync every page tap anyways it syncs periodically so the Kindle is storing that tap info and sending it all so it's not like this is just a factor of logging the data it gets sent for updating page position. [0]
[0] I think Kindle on Android is the worst for this. Sometimes I don't get the position synced to my Kindle even 30min+ after leaving the Kindle app. Seems like the way to guarantee the server gets updated is to either exit the app or to return to the library.
Regardless, I fail to see how storing each update is more invasive than just storing the last update when the same number of requests are sent to Amazon.
The whole history builds up a picture of your habits and reading times where only storing the last gives just that the last time you read the book. Think about the difference between storing the last place you were vs everywhere you've been. Individual data points may be innocuous where a collection of the same data points isn't.
[0] I think Kindle on Android is the worst for this. Sometimes I don't get the position synced to my Kindle even 30min+ after leaving the Kindle app. Seems like the way to guarantee the server gets updated is to either exit the app or to return to the library.