You think that your email is secure and encrypted because you are using Mailpile. Instead, you management has hacked in a back door to spy on your communications.
I don't think a license can solve this problem. I doubt someone who's willing to spy on people is afraid of breaking an open source license.
However consider what they are building here: a mail client that purports to securely send and store email. Imagine you are working for a company that uses Mailpile. You think that your email is secure and encrypted because you are using Mailpile. Instead, you management has hacked in a back door to spy on your communications.
As you mentioned, perhaps someone should push back. I'm not involved in Mailpile, but it seems that this is exactly what they are doing. They are drawing a line in the sand and saying, "You must show your users any changes you have made to the system because otherwise how are they to trust it?"
It's clearly a tradeoff and also clearly not an easy decision. Do you give up a little bit of your goal of protecting users at the benefit of having wider exposure? Or do you refuse to compromise and suffer from potentially having less adoption? It doesn't seem that they took the decision lightly.