FOSTA-SESTA is all about "knowingly" participating in "sex trafficking". a) I'm not sure that you can knowingly have an idea what's in encrypted blobs, so I don't think this would enable E2EE; they seem orthogonal. b) They're not policing a communication medium, but local files. Sex trafficking almost tautologically implies some kind of communication, so the target being local files instead of, say, local storage of apps like messenger and signal seems a bit off as an explanation.