UPDATE by Charlie: The issue for Mazzaro, it seems, was just as David Todd described it in the podcast -- Mazzaro was slated to make more than the league minimum. Not much more, at $950,000, but even that seems to have been a deterrent. And as I mentioned on the podcast, things like a shiny relief ERA just don't matter to most big-league GMs anymore, and Mazzaro's skill set isn't rare.
Given that the Pirates got Mazzaro through waivers, it's no surprise he accepted an outright assignment. If no other team wanted him on their active roster, then he probably had just as good, or almost as good, a chance of making it back to the majors with the Pirates as he would have with another team. Clint Hurdle's willingness to lobby for Mazzaro to get a shot with another team probably built some goodwill. It's nice to have him back.