AT&T and Verizon are two of the most powerful companies in the US, and have massive lobbying power. The notion that those companies were compelled to sign on to huge espionage programs, with their chiefs being fully aware of it - but yet somehow Facebook / Google / Microsoft / Apple etc. did not have to sign on to anything related to Prism (which has been openly admitted to exist by the Feds), nor were they aware of anything, is just about impossible.