There is an ongoing cost for email to work (static IP, DNS, tunneling for traffic) - this is why there is a subscription. It's not very complicated or malicious.
If a customer doesn't want a subscription, they can still use the Helm on their local network and get access to all their data. Running without a subscription is not something we currently support because of a lot of variables that can prevent email from working consistently, creating a heavy support burden (which is costly) along with bad customer experiences. It's best for people wary of a subscription to go down the DIY path (and we end up with a lot of DIY switchers once they realize how much effort it is).