Wow, lot of really diehard fanboyism in here...
Anyway, iPad makes sense as it is a platform with basically one format to orry about, one screen ratio, and one distribution channel. A lot of mobile stuff starts on iPhone/iPad only as it is cheaper to develop. The problem with developing for Android (both phoen and tablet) is the really large number of different handsets and subversions of the OS. Both handset makers and even individual networks customise elements of Android to their devices, making for a dizzing arrat of possible devices you might end up needing something to run on. For Apple it is considerable simpler.
This is basically an aspect of the old apple versus x86 argument. You can go for a closed ecosystem where everything is made my one manufacturer and get homogeneity leading to (generally) higher ease of use and better integration but at a higher price and with less user choice. Or you choose an open ecosystem which si lower price and has more user choice but leads to much more complexity for users and developers to handle.
What this means now is economically ya developer can give you a fixed price for developing for iPad, plus you know how to sell the product to people. Thats a lot harder on Android for now, so people tend to start with iOS then expand to Android if it is popular.
Also, is seems figures from March show
Apple has 73% of the tablet market, which makes them the best to start with economically. If you were targeting smartphones not tablets then you'd really need to pay attention to Android, and maybe in fact smartphone editions of codexes, reformatted to be easily usable on the move would be a good idea, but that is a different product that requires new layout. Moving from paper to iPad is a relatively simple design task.
Kindle makes less sense to me, as only the kindle fire has colour graphics and the stuff they are releasing is the graphics heavy stuff not the text heavy BL stuff (which already runs fine on Kindle anyway).
I guess if this is popular you may see Android version soon. For Android fans, this means you want the iPad owners to get buying to show the concept is popular!
[I use a windows pc for gaming and media, a mac laptop for work and a raspberry pi running Linux as a media centre, fanboyism is pointless]