I said in my comment “I’m sure the writer would have thought some curious passersby would stop to read it.”

My point is therefore not that it’s wrong to read the letter, but that there’s a big difference between reading a random letter you find and going “oh thats personal I should respect it” and going “wow I bet I could get internet points if I take a picture of someone else’s response to the loss of a loved one and post it on Reddit.”

It’s similar to how I believe it’s wrong to take pictures of fatal car accidents, or people in extreme emotional distress. It’s a private thing that, while seen in a public place, should be respected and left alone. Maybe your morals don’t line up with mine and that’s fine, in morality there is no one right answer.

I’m sure you can agree regardless that while the writer surely knew others would read it, it was not meant for any of them.