Well, first reason is because people leech of my bandwidth all the time, and I usually don't care. If it's something big I'll talk to the webmaster about it, but otherwise I let it slide.
But primarily, it's just too much effort for something that's (so far) pissed off all of three people. Like I said, this stuff is usually through geocities accounts or personal websites. But since I don't make a dime off my site, and it's just a hobby, to police my forum and question the source of every single image that somebody linked to would take all damn day. And since I have a real job and a real life, I don't exactly have the time.
Like I said, if anything pisses anyone off, they can come to me, and I'll remove it. Simple as that.
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Sharkey
Head Badass
http://www.badassmofo.com