> Plagiarism is an accusation which can ruin a career.

There's a difference between ownership of an idea, and plagiarism. An example of trying to 'own' an idea would be:

  1) Person A creates an idea
  2) Person B independently creates the same idea (or
     something extremely similar).
  3) Person A attempts to assert control over the idea
     by dictating what Person B can or cannot do with it
     based on the fact that Person A 'got there first.'

An example of plagiarism would be:

  1) Person A creates an idea.
  2) Person B copies Person A's idea, and attempts to claim
     independent creation (or attempts to claim creation prior
     to Person A).

The real difference is that with plagiarism, Person B is committing a fraud about the source of the idea. It's more about authorship than anything else. Even in a society where you couldn't own an idea, you could still have authorship as the first person to think of something.