Same here! Very interesting until I realized it's subscription based. Apps should have a one time cost and maybe costs for major updates. I can't even see the reason to have a recurring fee, why?
Because it's not an app in the regular sense, it's an email filtering and tagging web service. You'd be paying for their servers crunching your emails over the time you were subscribed to them.
Edit: From their FAQ:
Mail Pilot is more than an app; it's a comprehensive online software
service. It runs on servers, and delivers and syncs content between
every Mail Pilot app that you might use.
You can access Mail Pilot through a number of desktop, native
mobile, and web applications. To make this possible, Mail Pilot adds
services to your existing mail accounts that it syncs between all of
your devices. Our servers store metadata about your messages (not
the messages themselves, however!) to bring to you the reimagination
of email that so many have come to love.
Mail clients can get away with charging one time. Our apps are all
free, but the service is paid for, much like other online
productivity and email enhancement services.
Edit: From their FAQ: