Doesn't matter if people were reading them in the 90s or not. The most recent book came out in 2011, and the show started in 2011. I know they're massive books, but I think people could read all five in three years if they were really that concerned.
Like I said, I don't go out of my way to spoil people, and I've even asked to have things hidden that aren't out-right spoilers, but could still spoil some people. But at the same time, I don't think people should expect to be coddled about spoilers. Especially when no one is directly spoiling you. Shaibana's example is someone being troutty. Carl's example of just reading YouTube comments is no one's fault but his own. People are allowed to discuss it in open forums. It's a 20-year-old series. And even if you just consider the interest since the show started, that's still three years. If in 2008 I had heard someone complaining because someone told them Snape killed Dumbledore, I would've said tough titties.
tl;dr I'm not in the habit of spoiling people, and I try to not be a dick and protect people when I can (e.g. having mods spoiler things, putting spoiler tags on my own posts, discussing things on Facebook IM instead of making statuses about it when I COULD), but I don't feel bad at all for people who are getting spoiled. I'm not saying someone has to read the books if they don't want to, just that the information is out there and you are in control of what you know.