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Why must you choose negative terms to describe copyleft?Because there's an unfortunate kernel of truth to it?
From the standpoint of people using any other license, copyleft code (intentionally) causes an undesirable side effect of attaching its license terms to your code base if the slightest bit of it is merged in, whether through a developer's carelessness, ignorance, or otherwise. And the effects may not be noticed for quite some time. It takes vigilance and developer education to prevent this from happening. One struggles to think of a term that doesn't have pejorative connotations to describe this effect. I have yet to see anyone suggest a credible alternative.
(And please don't suggest "protective"; that's so transparently doublespeak that it's surprising that it ever took off. Things that are protective don't just reach out and latch onto other things they are accidentally brought into contact with.)
EDIT: minor rewording for clarity