If A invents a machine, B copies it and makes a million, B is 
benefiting at A's expense. If A copies a passage in economics and B 
copies it, B might make a million. But he does not make it at A's 
expense.  Much more than a passage or even an entire article must be 
copied for B to hurt A to any significant degree.

Also, if B is free, under the law, to copy A's invention, it harms 
SOCIETY because the prospect of having an invention copied reduces 
the incentive to invent. But what would happen if economists were 
free, under the law, to copy each other's words without citations? 
Would anyone be harmed besides the members of the economics 
profession? And would the harm to original writers be greater than 
the gain to the copiers?

Pat Gunning