You don't even need to back door it. WhatsApp already implemented restrictions on forwarding of messages, which shouldn't be possible if the encryption worked. Of course they didn't need to: you can just include a "forwarded counter" in the message and enforce it on the client. That trick can be done for any rule they wish to enforce.
As do many of their employees.
You don’t even need to make encryption illegal. You just need to backdoor it. Trivial for one or two well-placed developers.