>"Maybe they found that the vast majority of people that add a chapter wanted to buy the book, so they made it the focus."Even assuming that most people wanted to buy the book when they opened a book sample's menu (which is unlikely, since you can't even see the book's price in that menu), this does not warrant changing the default selection from a non-destructive, repeatedly used action (for every other document type) to a "destructive" action (for book samples).
You just don't randomly change the order of menu items when people expect to see the same order, and you most certainly don't change the default action from something that is used often and is harmless to something that is potentially harmful.
>"The ONLY solution to the problem of
accidentally order would be to require
users to go through a multi-step process
of confirming the order"
This is a false dichotomy. The choice is not between "make ordering the default action" and "make it absolutely impossible that people will ever order a book by accident." You can vastly improve the situation without solving every edge case.
The solution is simple: don't make buying the book the default action. If people have to consciously switch to a "buy book" menu entry, accidental book orders would very likely be cut down to a fraction of what they are now.
>"User experience, of which UI design is
just one part, is much more important"
I don't see how the distinction is relevant in this context. Are you suggesting that accidentally ordering a book is a good user experience? :-)