I don't care what the authors say, I will not refrain answering a question because someone somewhere requested people not to answer this question.
The text of the question belongs to the author of the book, and they may request that the text not be posted; in fact, that is automatically the case due to copyright law. The semantic content of the question does not belong to anyone, and anyone may ask the question, whether they first read it in that book or they made it up on their own. The authors of the book do not have a monopoly on the question.
Acknowledging the source of a question is required by academic traditions. Refusing to answer questions is very much against academic traditions.
Even if you're favorable to the authors' request (which I'm not), consider this from a practical angle: let's say someone wants to ask a question that is an exercise in this book, and they're aware of the policy. They can easily work around the policy by pretending to have come up with the question on their own. It's unenforceable.