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.