Not to take this off topic, but for an explanation of how British TV networks differ from US TV, read this
http://www.tv-ark.org.uk/internation...filiation.html
What does it all mean? Suffice it to say that LOCAL affiliates, and thus, LOCAL advertisers have a lot more control over the content of TV programming. Plus there is the now ingrained belief that Americans who want to look at naked bodies will PAY to do so - through premium cable, pay-per-view, and DVDs - and that there is no incentive in simply giving it away for free over the airwaves.
For a look at the long - if mostly accidental - history of nudity on US television, read
http://www.tvacres.com/sex_nudity.htm