It's a request. You can always so no. I don't see why this is an issue. The FA has to at least ask you or they could easily have a complaint against them. There's nothing wrong with asking people anything in this world. Most of us exist because at some point someone asked a question to someone else for which they usually answer no.

Were guy across the aisle so concerned about light, he could book a window seat or arrange for some travel gadgets that would help deal with the sun in his eyes or on a screen.