The mandatory arbitration clause is designed to protect Roku from a suit by its subscribers. In this litigious age this is understandable if only in a cynical self serving way. That said, it is anti-consumer to the point one could justifiably call it an unethical business practice.
The rub here is you bought and therefore own your Roku device but that device depends on an ongoing service from Roku to fully function. So to what degree can one side impose terms on the other is the point of contention. For example this is quite possibly not enforceable on a Roku box that just launches Netflix for you. The only reason to make you sign in is to serve you adds. On a smart TV maybe the lines are more blurry.
In either case I wont be buying any more Roku products unless this provision is withdrawn, other similar devices lack the onerous and shocking arbitration clause.