When you start looking into terrorist incidents, however, you find that ransoms are indeed often paid (even by governments, even by the US government). They try to keep this secret; sometimes it works, sometimes not. One example is Iran/Contra, but there are many others (e.g., http://www.foxnews.com/story/0,2933,48779,00.html , http://minnesota.publicradio.org/collections/special/columns... , http://www.commondreams.org/view/2011/03/18-10, http://en.wikipedia.org/wiki/Maritime_history_of_the_United_...)

Secretiveness, in this case, is a mechanism to avoid the slippery slope. As a sister comment to your comment points out, often other effects will step in so we don't go all the way down the slippery slope, and the "secret ransom" is one such effect.