While Plex does indeed have a paid tier, and they do charge for (some) of their apps, it's pretty much a free product. The paid tier gets you some of the newest features and access to the most recently supported platforms, but beyond that, the free product is not crippled in any way. None of that is my point.
Plex and Kodi started from the same project, and although the two products are not 1:1, they're pretty goddamn close. Plex is available in the Fire TV App Store but Kodi is not. Amazon could have believably cited the approval process to keep Kodi out of their app store (you could only sideload it as it was [and still is]) - hell, they haven't let a Plex update through since I bought the box - but they chose to go this piracy route. Pretty disingenuous, it seems to me.
Also, Plex rules. Kodi is a great product, but Plex is very full-featured, if less extensible, and is a lot easier to support.