Plex isn't necessarily piracy.
Even if your area allows it for "backups" or interop, you're making it available in multiple locations at once. I'm sure it wouldn't take much to demonstrate this was some sort of performance or other act explicitly disallowed by the EULA. But I'm sure you're just sharing Big Buck Bunny ;)
My point isn't that it's illegal, only that it's disingenuous enough to suggest it is legal. Plex definitely lies in a grey area of tolerated home use, with features that turn it into a piracy nightmare.