From the article:

  “My phone number is public on the web. No need to implicate WhatsApp,” 
  one user told Threatpost, explaining that Click to Chat was convenient 
  and made it easy for his site visitors. “I did it to make it easy for 
  people to contact me. Surprisingly, I get very few spam calls,” he said.

  However, others were unaware their numbers were public.

  “No I didn’t mean to make my number public at all,” one user told 
  Threatpost. “I set up WhatsApp for my business so people should 
  text directly without getting my number.”

I think this illustrates the underlying problem nicely, which is imho that people misunderstand how the "Click to chat" button works.