The Post/Guardian articles didn't claim that PRISM was being used officially to spy on Americans. They pointed out that its stated purpose was to spy on people thought to be foreign (probability of 0.51 or greater that the target is not a US citizen), and mentioned that some American citizens get spied upon "incidentally" and that this data is supposed to be removed - but also that it's not considered a big deal if American citizens are spied upon.
Additionally some of the communications of foreign targets can involve a US citizen, even if they're not the target of the spying.
Remember also that the intelligence community uses its own private definitions which make it easier to misdirect the public.