Actually it's becoming less unusual which is what's troubling. Just like the ipad mini didn't get a retina display, these ipads aren't getting the TouchID feature. I know they sound like little things and maybe not everybody cares about them, but it shows me that there seems to be more of a disconnect in development throughout Apple than there has been previous.
This makes the assumption that there were a sufficient quantity of parts for Apple to include in these products. Given that the upcoming quarter is contains Christmas and it's usually the best quarter for most hardware companies, it's possible that there were not enough TouchID sensors available to include in all of the product lines. Keep in mind that this is a company that is expected to sell tens of millions of these devices.
Last year they sold slightly more than 20 million iPads and nearly 50 million phones. Although not all of those are the most recent model, it's suspected that many are. If they were to include the TouchID on all of their devices, they would probably need at least 60 million sensors. That's a lot of anything to produce, especially when the first product to use them just started shipping a month ago.
I'm sure that they probably wanted to include this feature on their iPad line, but realized that it wasn't going to be possible to do so, at least not without limiting their production. At the end of the day they need to keep the stock price up, and that means having another blow-out quarter, which means they need to ship as much product as possible.