It seems like multiple things are being conflated here and I'm not sure what the reality is because I've never used Plex.
Some people claim this has something to do with Plex needing to pay for NAT traversal infrastructure. Okay, that seems sort of silly but at least there's the excuse that their servers are involved in the streaming somehow.
But their wording is very broad, just calling it "remote streaming." That led me to this article on the Plex support website, which walks people through setting up port forwarding in order to enable "remote streaming"! So that excuse doesn't really seem to hold water. What exactly is being paid for here then? How do they define what "local streaming" is?
Being able to sell FOSS is one of the freedoms "free software" refers to.
Honestly though I think the thing that struck me the most and I found kind of scummy was their "value statement" where they were advertising the OS by comparing it to the prices of the proprietary software is includes alternatives to. You misreading the website wasn't an accident, they designed it in a deceptive way IMO.
If they were more honest about it, I wouldn't have any problem with them charging for the convenience of having everything pre-bundled. Of course you could set everything up yourself, but Linux is notoriously finnicky. People want a complete experience, they want support. They want the slick branding.