Liability.
US based programming has to be so careful - they are all afraid of people imitating them, getting hurt and suing the networks etc.
Even though it is highly unlikely, they (the networks) are afraid to step out on a limb - Everything has to first go through legal, so can you imagine what some high-paid legal expert is going to say when presented with the idea of cars being driven dangerously on TV with their consent?
And even WITH legal approval they get paranoid, especially with live TV - look at Janet Jacksons tit - the half-time show was approved - but the artists slipped in a tit-job for free...
LOL
... but then again they let these recorded freaking "reality" shows on TV with only a lame "Don't try this at home" disclaimer - so who knows?
Time for a Cable Channel to step up and do the dirty work I guess... but which one?