Why? It’s simple: Facebook has taken over WhatsApp and uses all contact data of WhatsApp users. Data of people who never were or never wanted to be on Facebook was flushed to Facebook this way. Unasked. You don’t need to know much more than that to uninstall Whats App immediately and simply make normal phone calls and send pictures by e-mail like before.
A slightly longer version is available in Stoffels Blog.
Or just read netzpolitik.org and set up a recurring donation order there.
Or simply use Threema. Quite the same as WhatsApp – only in good and secure.
Update December 2018
And for those who still don’t understand what the problem with Facebook is, we recommend this list of data scandals on Facebook in 2018. It’s long, though.