Digital libraries are treated differently because they transmit the book by copying it from one machine to another, physical libraries transmit the book by handing it to you without making any copies, and copyright law restricts copying. DRM doesn't have anything to do with this, the fact that you use the word 'library' to describe the former doesn't have anything to do with this, and the fact that IA didn't make a new copy unless the recipient of the old one had deleted it doesn't have anything to do with this: unlicensed copying outside of fair use is copyright infringement. The remedy for feeling that this is unfair is calling your congressman to change copyright law, not violating it on purpose and then complaining when you get sued.