OP, were you flying from Madrid in BusinessElite? If so, this could be a fun coincidence: I actually spoke with a flight attendant and made this request on a flight from there, so I might be the person you're complaining about if so.

In my case, the flight was timed as a 10am departure, Madrid time, and a 3pm arrival into ATL. While that might seem like a daytime flight at first glance, I needed to sleep on the flight to reset my body clock for jetlag: my final destination timezone wasn't EST, and from the perspective of PST it was my only chance to actually get a good night's rest.

It wasn't my choice to be on that flight, by the way - my original CDG-SEA flight got cancelled, and this was the only one Delta could rebook me on due to the way the departure times worked out.

I try using eye masks, but they don't block out natural light very well at altitude: the way the sun was shining into the aircraft, the light from the two passengers' windows that were fully open in the cabin hit my seat at full blast. Natural light is a powerful signal to the body's sleep cycle, and it's really difficult to get a few hours worth of solid sleep when the cabin is bathed in it. Invariably, when I wake up mid-flight, I look up to discover someone has opened their shade to look outside, and my body interpreted it as sunrise...

During domestic flights, I have no expectation for windowshades - they're controlled by the passenger in the window seats. ^

On international flights in J, though, almost all of them are timed as night flights for somewhere, and it's likely someone needs to sleep. My typical etiquette when seated at a window is to lower my shade and use the reading lights while the cabin lights are dimmed (e.g. from first meal service to second meal service); before and after those times, it's totally reasonable to keep the windows open.

This is a long-time topic of debate on FT; it usually degenerates into arguments about who has the "right" to take one course of action or the other. I look at it differently - it's just a voluntary act of courtesy from one passenger to the others sharing the cabin.

Different airlines have different takes, too; many of the Asian carriers I've flown pass through the cabin and politely ask others to dim their shades so others may sleep during the middle of the flight. I think that's a nice touch - someone who isn't trying to sleep to beat jet lag might not even realize the impact they were having on the other travelers in the cabin. Some European carriers I've flown just forcibly reach across the seat and shut everyone's shades, which I don't support.

On my particular flight, I asked the flight attendant because there were multiple windows open and I felt it would have been inefficient and disruptive to speak with everyone myself. I actually chose that route partly because many of the Seattle based crews do that automatically, and I actually thought it was part of the DL service standard as a result - but mostly because I was trying to be courteous to the passengers with their windows open and minimize the disruption of being approached by a random other passenger. In the case someone else cited where someone has a legitimate medical condition where they need the shade open, I feel the F/A is better equipped to have that discussion than I am.

Unfortunately, neither of the two passengers who wanted their windows open were willing to be courteous and lower their shades; fortunately, the flight wasn't full and the flight attendant was able to offer me an alternate seat in the rear of the cabin where I was able to get some sleep.

Whether or not you were on my flight, hopefully this offers some perspective and you'll reconsider your stance on a future trip.