There's entire categories of entertainment media that use "unfortunate things happening to strangers" for comedic effect.
I mean, keep in mind I was fourteen when this happened, and fourteen year old boys are very often assholes and I was sadly not an exception to this.
I guess I just found it funny how much of a reaction people had with it. I liked the Harry Potter books too, I was reading them like every other fourteen year old was, and the plot being spoiled for me didn't really bother me very much, cuz, you know, it's just a book. Some people really got upset.
Again, definitely a dick move to do that, and a dick move for me to find it amusing. Kids are douchebags.
Also, there's the assumption that it was a real spoiler. It's not immediately verifiable. What if people were yelling "Snape kills Harry"? Why did the people in line assume the guy yelling into the night was being truthful, or that weirdly cropped images of pages weren't just photoshopped?