[PBondurant]

No, this isn't an example of Godwin's law being used as a tactic to end discussion; rather he was using hyperbole - a rhetorical device - to make a point. The slavery reference is not used to suggest that 'non-free software is morally equivalent to slavery/nazism' etc but rather is used as an example of a clear, strongly held moral position in order to emphasise the (possibly more subtle) original point. The point of hyperbole is to further elucidate, not close down.

This misunderstanding tends to be caused by (1) treating the rhetorical device literally rather than as an (intentionally exaggerated) analogy, and (2) - as in this case - by assuming that the argument being made is trying to draw some moral equivalence between the analogy and the original example. It isn't.