Nothing is obscene according US law until it has been convicted in a US court of being obscene; and again, only that particular item(s) that have been convicted in that particular court case. When enough cases of apparently identical material are declared guilty of obscenity, it is designed to produce a "chilling effect" were other producers/retailers back off from supplying that particular community with material of that nature. Obscenity laws are normally punished as major misdemeanors or minor felonies of 3 months to under several years of imprisonment anyways, targeting the seller or producer 99% of the time - virtually never the purchaser (true photographic child abuse porn the exception, following the Feber ruling).
How come the millions of American men of my Dad's generation are not in prison for subscribing to Hustler magazine with its monthly Chester the Molester cartoons?
How come the tens of millions of Americans are not in prison who download and share Bart Simpson cartoon porn featuring Maggie & friends over the past decade or two?
How come the million of Americans and most every adult DVD retailer in America and the Web is not in prison who sell for years DVD's featuring over 18's cosplaying preschoolers, gradeschoolers and middleschoolers having XXX intergenerational sex in the number one selling XXX DVD's in America - This Ain't the Bradys XXX, Marcia, Marcia Marcia XXX, Ain't the Bradys XXX Pussy Power, Not the Cosby Show XXX, Not the Partridge Family XXX, This Ain't Married with Children XXX etc.
Seems the Internet for the recipient is not interstate commerce.