SPEWS does not block anything. They simply provide the blacklist which email admins then elect to use.
Frankly I dont see the problem from their point of view. Spam sucks and we get our share of it here too, but I and the managment here are not going to block spam in a wide sweep as SPEWS does. That said I should refrain from bad mouthing those who elect to use it. They may have very legitimate reasons for doing so.
I'm hoping to post a counter arguement in favor of SPEWS. I want to be fair about this. I'll be honest, my knowledge of email administration is small. As long as it's working that's all I care about.
One question that still hangs out there for me is where is the costs? How does spam cost us millions? I'm not trying to be argumentative, but honestly asking what I feel is a legitimate question.