If the damage I accidentally did to a book does not affect its readability and is mainly of a cosmetic nature, why am I required to pay for the
This is not a hypothetical question. I spilled something on one page of a book I had checked out of the Millard branch. The accident did not affect the readability. A librarian told me that I owed the price of the book. When I questioned the charge in view of the minimal damage, I was told by the librarian that, well, the book could no longer be checked out (even though I have checked out a number of books from the Omaha Public Library which had far worse damage than what I did). Another librarian (who I think was a supervisor) was called in and told me that, well, the price on the book cover was not the price paid by the library. I took that to mean I would be charged a lesser price than on the book cover. The second librarian then went to a computer and after a few minutes told me that the charge would be $30 (not the 25.99 U.S. retail price or the price the library paid.) He did not respond clearly to my question of why I was being charged the Canadian price. Neither of the two librarians filled me with confidence that they knew the library's policy on this kind of issue; in more than 40 years of using the Omaha Public Library this is the first bad experience I've every had. The good news I guess is that I did get to keep the book. (An unrelated question: Why in god's name is my age required in order for you to give me your "best response"?)
Sadly damage of a cosmetic nature does affect people's enjoyment of a book, so we usually have to discard damaged books, unless the damage is very slight. I am sorry that you have come across damaged books! When we discover it, we do withdraw them, but unfortunately sometimes we don't catch it in time. The charge for replacement is what the library pays to replace the book, which usually includes the cost of cataloging, security stripping (putting security strips in the binding that beep if the book has not been checked out properly, to discourage theft), and barcoding--that is why it is usually more than what you see the book selling for on Amazon, for example. I am so sorry that you had a bad experience, and hope it will be the last you ever have!
The system asks for your age because it is helpful for certain types of questions--for example, those involving book identification (where it helps to know what age level the book might be) and a few other types of questions where it might be helpful to know if a child or teen is asking the question.