SOPA/PIPA were never attempting to strike
plagiarism, rather, both are targeted at piracy.
Plagiarism is defined as taking of someone else's work and proclaiming it as your own. A few individuals doing this to a corporation would have a negligible impact - the corporation's resources are easily used to disprove the claim and discredit the forgers.
The financial incentive to fight plagiarism is incurred by the fact that someone who claims to hold intellectual copyright on a product can then sell it as their own - creating unnecessary competition and undermining the corporation.
Piracy, on the other hand, is quite the opposite of plagiarism.
Plagiarism poses a financial threat because copyright forgers may attempt to sell the material as their own, but piracy is simpler - and thus has the potential to become more widespread.
It requires time and effort to fabricate a claim on something, as in plagiarism. The forger must also convince many others if they are to make a profit.
Piracy doesn't involve any
formal justification, such as claiming ownership of a product. All piracy requires is the acquisition of the material in question.
Internet piracy is the subject of such dedicated harassment for two reasons:
1.) The internet is far less regulated and unobserved then traditional methods of transportation, such as overseas trade.
2.) Traditional piracy is self-regulated by the fact that each unit of product must be, at risk, acquired. This meant that often pirated goods were not given away, but resold at discounted prices to allow the pirate to recover the costs and risks of obtaining the pirated product. Internet files, on the other hand, may be distributed freely because they may copied endlessly and without risk. This takes away the operating costs of piracy, while opening the market up vastly at the same time, as end consumers will get the product for free.
As a result, internet piracy poses a much bigger threat than conventional piracy ever did to profit.
The discussion should be not about plagiarism, as that is highly uncommon in virtual products and is not the focus of SOPA/PIPA.
While the distinction between plagiarism and piracy may seem obscured if people discourage both simultaneously, they are actually quite separate, especially in this context.