People buy fakes because they don't know or care. They don't know that they're supporting criminal activity, or they do know and don't care. Many people see knock offs and a victimless crime, because the harm isn't something you see first hand. I think there are some people also don't know enough about brands to know if something is a knock off. They see a bag they like and they buy it, with no ill intention at all. This doesn't mean there isn't harm caused, but it is unintentional. I also think there are people who desire the brand, the prestige of the brand, without paying for it. They know it's a knock off, but their desire for the brand name over rides any ethical questions about getting it illegally. They figure if the companies did not charge the dear earth for the privilege of wearing their name there wouldn't be knock offs, fakes or counterfeit goods. There are many reasons people buy fakes.
What really irks me is the people who brush off the importance of it being illegal. Like it or not, it's against the law. The criminal groups that produce and sell fake goods also support terrorism, human trafficking/slavery and illegal drugs production and distribution. "According to Gieschen Consultancy, a Canada-based counterfeit-intelligence monitoring service and consultancy, more than 5,000 incidents of counterfeiting and piracy were documented from October 2004 through mid-January 2006, totaling more than 1.41 billion seized counterfeit items worth an estimated US$ 4.13 trillion." That's a lot of money taken out of the economy and put into the hands of criminals. "According to the New York City Police Foundation, an estimated $1 billion in New York City alone is lost in tax revenue from counterfeit purchasesmoney that would otherwise benefit the city's community."
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http://www.straight.com/article/new-york-knocks-off-fashions-big-names
Another great source:
http://www.iacc.org/counterfeiting/counterfeiting.php