> Can’t Talk‘s premise is simple. All it does is automatically reply to people when you aren’t around, making use of the quick reply (RemoteInput) API native to Android. […] It’s a paid application if you want to support more than SMS and calls, such as WhatsApp, Slack, and a lot more.

> DirectChat is another useful application. It works just like Facebook Messenger’s chat bubbles but it supports a lot of different applications too. It feeds your chats into another window where you can then reply back as well. Its most popular usage was probably with WhatsApp.

I think the "Cease and Desist" letters are being sent because these two specific apps, unless there are others not mentioned by these article, are making use of APIs that are not directly part of WhatsApp but interact with it in a kind of direct way.

I wonder if the letters were sent directly to the developers or via Google Play.