The existence of other legal authorities does not imply that tech firms are voluntarily working with NSA to help NSA harvest their data transiting the Internet.
It doesn't even make sense anyways; what does Facebook have to do with surreptitiously tapping into a router in Belgrade or Quito?
Whether cooperation is voluntary, or not, is not the issue. Nor is tapping the carriers, with or without their cooperation. The article says that cooperation can be compelled.
The issue is that some of the participants in PRISM denied providing "direct access" to their data. Some people here are saying those denials are meaningful when we do not have a complete picture of how cooperation is compelled.
It doesn't even make sense anyways; what does Facebook have to do with surreptitiously tapping into a router in Belgrade or Quito?