If you don't do this, acknowledge that you are renting a reading experience -- and pay what that's worth to you, as opposed to the price of owning a copy of a book.
You've missed the point. Something like a Scribd or Oyster subscription use an app and its own eBook format. The books are cached or streamed, not downloaded permanently. You are paying to read, not to own.
On Android, the FBReader.app is quite good, however you probably will want to tweak the options to get it to look as nice as the Kindle.app looks "out of the box."
On iOS, the Bluefire.app is the best of the unaffiliated apps, but I preferred to open .mobi files in the Kindle.app.