People make too much of the name. If you were involved with a top-secret government surveillance project, would you ever mention the name outside of the NSA, even to companies you're partnering with? It's likely that until now nobody outside of the NSA had ever heard of "PRISM", which is why you get these blanket denials from the tech companies supposedly involved. To them, it's "The government is coming to us with requests for data on our users", not "Help work with us on this top-secret project named PRISM".