The problem is that Amazon were removing content that had been illegally sold to american users... illegally, as in America the seller did not have the legal right to sell the product.
Therefore, the purchaser of the ebooks were "buying" essentially "stolen" products, and have no more right to the product than I would if I had bought a stolen car.
Additionally, as has been pointed out, you do not buy the ebook, but merely pay for a license to access the content of the ebook, and as such Amazon and the rights owners could remove anything they wanted as long as it was within the terms of the license agreement. An interesting point here is that a license was not sold to the ebook, as Amazon had no right to do that, and so no license agreement was in force, and as such the purchaser had not agreed that Amazon could remove that particular ebook (unless of course there was something extra in the Kindle EULA along the lines of "Amazon reserves the right to delete whatever we want").
The only reason this turned out to be a problem for Americans is that the copyright ownership laws in America are very much in favour of the producer... they have ownership for a LOT longer than over here in Europe (it's only 30 years in the UK). Personally I think 30 years is long enough to recoup enough income from the effort needed to produce something. Think of it this way, if a mechanic fixes a car, should he be expected to receive income from that for the next 100 years, or to put it another way, if a singer in 1940 had a single hit should they expect to keep getting an income on that recording even now even though they got paid reasonably for that single days recording ???