This is no different from the used/lent situation with books, CDs, whatever. The only difference I see is that in this case, which I admit might be a bit annoying, the developer can see the extent of second hand sales or lent games.
Really this is no different than me buying a book, reading it in a day and then selling it on or loaning it to someone.
I can understand the issue with piracy in the same manner as the issue with books being reprinted illegally for sale, but not second hand sales: they are a staple of everything owned in our society and I see no reason games should be exempt.
One thing to remember though, if everything does go digital lending and second hand might vanish as they require a physical object, so be careful what you wish for (although personally, if it was me I would allow lending of digital stuff from a goodwill perspective).
EDIT: the other thing I meant to mention is that with second hand sales you at least make the initial sale. Basically they made 2 million sales and 1 million people at least wanted to buy second hand or borrow who might buy first hand next time. That's pretty good IMHO for the QD brand.
Try to be reasonable... its easier than you think...
