An email address is for most people their single point of recovery for almost all of their online accounts, including services like Paypal. If you choose to trust an email provider, you are trusting them with access to your entire digital life, including finances. If you can't trust a service to responsibly manage your payment information (in the case of fastmail) how can they be trusted as an email provider? Someone could do far more damage to me with control over my email address than they could with access to my credit card details.