The illegality of fisting is a myth that started after the Meese commission report on pornography came out and the industry was worried about rampant prosecutions.
Adult industry lawyer Paul Cambria tried to "help" by coming up with his "Cambria List" of things NOT to shoot so that the US administration would refrain from prosecutions.
On this list was fisting, among other things such as interracial sex.
In fact it was Cambria who came up with the "rule of thumb", ie. not inserting the thumb with the four fingers to avoid having it called fisting.
None of the items on Cambria's list, many of which have since grown into siginificant niches within the industry, have been brought to court or successfully prosecuted - over even the supreme court ruled "community standards" interpretation, which is becoming more impossible to maintain with every passing case.
Actually if you aren't sure what niches are out there and would like a good run-down, Cambria's list is probably still rock-solid, check it out:
http://www.pbs.org/wgbh/pages/frontl...g/cambria.html