If the remote talks to a TV, I'm 99 percent sure that the interpretation of the commands takes place in the TV. (The remote is a fairly dumb device). On any TV that has been made in the last 10+ years, the firmware can be updated by the manufacturer, so they could always add a settings option that would allow the user to specify what each button should do.
The remote is smart and the app should be developed to enable these buttons for other functions. My two cents.
As others have stated in the thread, Roku receives $1.00 per assigned button on the remote. $4 per remote to subsidize the cost of hardware trumps your $0.02.
I'd pay $4 to be able to program them on my own. I'd pay twice that to have a say in a newly designed remote that was more intuitive to the touch in the dark. The Play Rewind and FF buttons could be set apart and easier to feel, and the pre-programmed buttons should not be jammed right up against them.
Your link in your post just takes you to the product overview. Where is the instructions on how to program the personal buttons? A little more documentation would be very nice to have with this device instead of having to search for it within the Roku app.