Unlimited purchase profiling and user habit tracking opportunities for Amazon? I bet they're pleased - now privacy is exchanged for convenience, you may as well post them your bank statements and medical records too...
They already knew the books and other products you were buying, now knowing which books are "borrowed" is suddenly a huge invasion of privacy? Do you use a credit card? They have the info on all those purchases. Order anything online from any site? And stores know who you are if you use a credit card or even a check. Borrow books and materials from libraries? Really any transaction that discloses identity has the potential for "tracking" although it has been that way for decades.