That is exactly the problem as I understand it, that "exploitation" is first understood as a negative description of an unjust situation, and secondarily as benefitting from the transfer of resources. While Marx does indeed believe that there is some injustice inherent in this situation, he is first and foremost trying to scientifically describe an economic process. And that's been kind of an effective semantic attack used against Marxist theory: that it is just an emotional diatribe against perceived oppression, and glossing over the actual economics of what he's describing.