It is a very complicated situation, but no, ISPs should not have the power to charge
anyone for specific kinds of internet traffic. Netflix and other video streaming services just happen to be an easy target for greedy network providers to focus on. As
@drunkpenguin said, it is a slippery slope. If they start getting extra money for Netflix traffic (no matter where the money comes from) they won't stop there. Next up will be YouTube (and other Google services), Apple, Facebook, Vimeo, Amazon.com, and everyone else they can identify as being associated with a significant amount of network traffic. Eventually internet providers would have systems and precedents in place to charge you extra for unthrottled connections between your cell phone and your home IP address to view your cameras.
You have a valid point, often once the door is opened it doesn't stop at only what was initially intended.
Roku is just a device that can stream from many different services, Netflix being one of them. I'm not sure if Roku has their own streaming service, but if they do, then it surely uses a similar amount of data per hour compared to Netflix.
Plex is a service you host yourself so typical Plex traffic doesn't touch your ISP while you watch it.
However, if you download the video from the internet in the first place (as opposed to ripping a Blu Ray disk or something), then it is basically the same as streaming the content once (possibly during non-peak hours).