I think most people realize that this is not an attack on any kind of open source community or a breach of ethics. Further, the unethical ones are those that worked on the project, knowing that it was not their api.
While legally the senders of the C&D letters are right, doing that might be an unwise move. Curbing your developer community in a hostile way may be a mistake in a longer term.
In a longer term, open solutions prevail. OTOH it might be not the moment yet for WhatsApp. Compare the stance of Microsoft in 2005 and 2015.
Just saying.