I do appreciate the attempts to explain by others, and the information you have all provided. However, it seems to be more mis-information and speculation which is just adding to the issue.
To clarify a few things...The Plex app is made by a *commercial* company (Plex, Inc.). Just look here to see that they charge money, and are an incorporated entity:https://itunes.apple.com/us/app/plex/id383457673?mt=8
Also, Plex does in fact develop and market a "Hulu Channel", which does pretty much the same thing that PlayOn does wrt Hulu. So I still don't see what the "legal" difference is between these two. I am watching Hulu on my Roku, right now, through the Plex Roku channel (developed by Plex). So the claims above about this being the critical difference just doesn't add up or jibe with the facts.
Lastly, I am pretty sure there is no such thing as "NDA clauses" in DMCA takedown notices, and I don't see why Hulu would care about PlayOn (but not Plex) enough to issue a DMCA notice to their *partner* Roku. DMCA takedown notices are pretty much never issued to a business partner...so I think that is just more false speculation.
So, unless somebody knows the real answer, and not just speculation, I think it would be best for Roku to explain what the difference is, and why they take issue with one product, but not the other, when they both ostensibly offer the same thing from a technical and legal perspective.