In an interview in Le Monde, I've found:
Le Congrès savait pour Prism, mais n'a rien dit.
Why “pour”? I couldn't find any example of savoir + pour in a couple of dictionaries I checked. Is it correct?
Savoir pour is a colloquialism. Note that this isn't from a newspaper article but from an interview. This construction isn't formal enough for a newspaper article.
Savoir pour translates into English as “know about”: “Congress knew about Prism but didn't tell”. A more formal way to express this in French would be “Le Congrès savait que Prism existait” or “Le Congrès était au courant de l'existence de Prism”. Slightly less informal than savoir pour is être au courant pour (“Le Congrès était au courant pour Prism”), but I still wouldn't use it in a newspaper article.
The preposition pour is used when the description of what is known is elided, leaving only the salient aspect. In contrast, de or que requires a semantically correct qualification of what is known. There are a few cases where there is nothing to elide in the description but in most sentences only one of the prepositions work. This should hopefully be clearer with a few examples.
Savait pour, in this context, means knew about:
I am not sure whether this is absolutely correct but this is widely used and admitted as such.