So the choice is simply: Pay X up front and promise not to tell anyone, then be left alone OR Don't pay, attempt to fight it in court (using lawyers/solicitors/barristers) and absorbing the cost of this upfront yourself, regardless of the outcome.
http://en.wikipedia.org/wiki/Corporatism
Some interesting quotes on the subject - http://www.goodreads.com/quotes/tag/corporatism
A few of the accused tend to stand up (grandparents who don't even own a computer, etc.) but I bet most just pay up to avoid the embarrassment of being dragged through the courts accused of pirating some smut.
Counter sue, entrapment, backmail, bullying, cartell - just anything remotely tangable then chuck it at them back. There rules after all