The problem is that this is fraud. People think if they are clicking a "buy" button online, what they pay for belongs to them. If someone then takes it away, they feel robbed, and rightly so. Saying "buy" implies ownership of the product; it's a contract term. If the fine print says "you don't really own this" then that deceives people, causing them to make different decisions, and the results can be negative. This does not just harm Amazon's reputation; it damages the reputation of ebooks in general. It makes customers feel that ebooks are unreliable, perhaps even undesirable; and that makes people less willing to pay for them. This is an incentive to copy them from unofficial sources, because the authorities can't burgle what they don't even know you have. It also makes life difficult for anyone who really IS selling ebooks on a "you pay me, and this thing belongs to you" basis.
Just in case you're wondering, that's me. You pay me for an ebook, and that copy is yours, just as if you pay me for a paper book that I mail to you. Robbing people's libraries is an abomination before the Lady.
And this is one of many reasons why I dislike ebooks, but even if I liked them for every other reason, such behavior would kill my interest dead, at least for anyone doing business on this basis. It can be hard to tell who's playing straight and who's fraudulent, though, so a few bad experiences and the whole product line gets crossed off. The risk isn't worth the reward.