For Amazon, when you buy an eBook, you are essentially given a license to use it. Amazon can revoke that - we've all heard of people moving to another country to see their entire library disappear, only (hopefully) remedied after enough hell was raised on some website.
As for Play Books, lets imagine you are a Android developer working on the next great thing, and you are an ethical person. Still, you can make a mistake. Suspension of an app is considered a strike against the good standing of your developer account: “Additional suspensions of any nature may result in the termination of your developer account, and investigation and possible termination of related Google accounts.”
You would lose everything, eMail, books, etc. So while I don't expect these problems, knowing there is a small possibility, with almost no chance of remedy by a human on the other end, dissuades me from investing lots of money in that ecosystem.