If implicit contracts are not valid then this means that if you DON'T wear that sign one day, for some reason, then it is perfectly ok for anyone to kill you. They can simply say "I didn't agree not to kill him, and he didn't say it wasn't ok." Sane people understand that life would be unbearable without such implicit contracts, and copyright law is simply one of these implicit contracts. Copyright COULD be implemented with a sign on the book stating "by opening this book you agree not to spread the content of this book to anyone" or if that is not acceptable then even sign an explicit contract with the seller that you will not spread the content of that book.

In short, none of the standard arguments against IP leads to a sane world because of the logical implications it has for other laws and rights.