See the last points in the article: run by activists, and would rather shut down than cooperate with law enforcement.
I don't know if proton is run by activists, but I do know they've cooperated with law enforcement by inserting code to log user requests when coming from a specific user. Plenty of articles about the court case, and it's also why they did away with their no-log policy.
Also, are their logins token based or username based and connected to the protonmail account?