Bad seems a little strong. But I think your point is valid, if rephrased as "potentially problematic". If you consider the common kind of scenario where a company wants or needs to integrate back end systems with one another, and the viral nature of copyleft licenses, you could quickly end up with your external contractor having the right to ask for code to completely unrelated internal systems that have nothing to do with Mailpile. I am sure that is not the intent of Mailpile authors, and I wonder if they fully understand that this might well put Mailpile straight onto a blacklist for a lot of companies.