Why do people have to be so rude when you try to help them?

Honestly, it blows my mind how someone can get mad when you try to help them (and not in a way that could be mistaken as condescending or anything).

My sister had the following posted in her profile on AIM:

Click on the link below to earn easy money... doesn't cost you a thing at all... only used it for one day and already earned $1,100.00...

And then the link she had following was a link to mailsmoney.net, where they claim you can cash out as soon as you hit $250,000.

I simply told her that the site was a scam and any site that offers that amount of money for paid to read is a scam and her response was:

so it must be a scam because of how much money it offers
well im still ganna test it out it isn't like im losing any money or anything

So I figured fine, maybe citing money as the reason it was a scam wasn't the best excuse, even if it was true. So I posted about five links where people gave their experiences with the site and said it was a scam, including a link to a site that listed all the scam sites. And she countered with this:

yea well unlike those people i haven't given them any money
i have a free membership

I was kind of like o_0 at this point, but I didn't mention it and merely said that if they paid money and didn't get anything back then she certainly wasn't going to get anything with a free membership.

And then she got mad and said fine, whatever and why do I even care so much if it's a scam or not and then she stopped talking to me.

Honestly, did I do something wrong? Because if I did, feel free to tell me because I really would like to know. I thought I was doing a good thing? I mean, I didn't mean to make her mad or anything but I thought she would want to know that it was a waste of time. I always appreciate when people tell me if the sites I'm on are scams so that I can go elsewhere. Maybe that's just me, though.