Well, the book has to be enabled for lending, and you can only lend a book once. I personally have no issues with it. IMO, it's like paperback swap. You read it once and you want to network with others to trade your reads. What can be wrong with that?
You'd be surprised how many people feel that lending is depriving authors of revenue, but yes, it's not much different to lending paperbacks.
However: the way ebook lending was conceived was probably that Alice and Bob are pals who both own Kindles. Alice lends Bob
The Girl with the Dragon Tattoo, and Bob lends Alice
The Da Vinci Code. There's nothing to stop Alice and Bob sharing their whole libraries, assuming they're enabled for lending, but only one person can have a given book at any one time. This is a nice cosy personal relationship that's precisely like sharing your print book libraries with each other, and this doesn't seem to be a problem for most people. Of course, if Bob owns a really enormous collection of books, Alice need never pay for a book ever again, but we probably don't worry about edge cases like that in the real world.
With Lendle, on the other hand, the library of books that are available for free lending is enormously larger. Now if Alice doesn't own
The Da Vinci Code, or know anyone with a Kindle who wants to lend it to her, it really doesn't matter - she can very quickly find a total stranger, Carol, who has it, and obtain it from her. As more members join, the library gets bigger and bigger, until you can obtain anything you like.
It's typical internet: you take something innocuous and use networking to scale it up until it's something very different...