By Kindle light do you mean the glow from the Kindle Paperwhite? Or do you mean one of the add-on lights?

I don't consider either rude just as I wouldn't consider it rude if you were using your laptop or watching a movie on a tablet.

I did think it was marginally rude when the guy across from me was playing a game with audio on his speakers rather than headphones. It didn't bother me after I put my earphones on but I had never seen anyone do this before.