What's interesting is that they have only sold 1.5M books, or 1.5 books per device. Amazon Kindle should be safe.
In addition to all the freely available titles in the Book store, everyone who downloads iBooks is also prompted to download a free Winnie the Pooh title. I expect virtually nobody selects "No" for this.
I would be very interested to know how many titles that have actually sold. I expect it's a fraction of 1.5 million.