People still have to buy it though. Apple isn't going to make crappy eReader so I don't see them getting into the game more seriously until the technology is better, and I don't see everyone carrying around a Kindle either. The Kindle has done enormously well so far, but the fact is that the average American buys only 1 book a year and 70% of them don't even read it[1], so I don't really see how it's going to go from a few million sales to a few hundred million sales. Sure Amazon sells an enormous number of books, but that's because almost all books are read by the same 2% of people or whatever. So let's say Amazon does make some move to ensure that it continues to control eBook sales, what exactly would that look like? Almost everyone who reads daily newspapers already owns a kindle or is on track to within the next couple years, and I don't think people who read magazines would replace them with a kindle. And the student market is just an enormous mess that I don't think Amazon is currently prepared to deal with. So what other captive population would they really be able to capture via a subscription subsidy?
[1] Possibly apocryphal, but probably true enough: http://www.humorwriters.org/startlingstats.html