well, those look more ebook readers than tablets, imho.
I agree that ebook readers also have significant trust issues, but compared to tablets have a couple of differences (note that most of it only applies to epaper devices, not to certain tablet-from-an-ebook-vendor).
* they tend to be more single-task devices, so you don't load your whole life on them (but I agree that "reading habits" on its own is already an important information that somebody may not want to share
* there are a few ebook readers (surely not the kindle, and I'm not sure about the nook, however, and those are the ones shown on that page) that allow you to use them without having to create a remote account, and they work even if not connected to wifi. Of course it's still proprietary software and you can't know that it's not trying to connect to random wifi networks to send your data by stealth, but that would require explicit malice.
* reading a book on epaper is quite different than reading it on a regular screen, and depending on your eyes it may cause MUCH less strain, as it's pretty closer to what one experiences on a regular book; here a laptop and a tablet would be pretty much the same, but an epaper screen can be a game changer.
Anyway, I suspect that the use of those devices in the image above has more to do with the fact that they want to show that those books can be read on the devices one already owns and uses, rather than suggesting that privacy-conscious users buy one of those devices in order to read those books.