I'd imagine its prohibited by insurance. Also think if the trigger gets pulled accidently once , then it will only fire 1 ball ( possibly at someone ) . If the marker if on full auto , and the trigger gets caught on something , then theres a chance it'll let a rope off , which is considerably more dangerous.
I can't ( and wont ) use the full auto on my timmy , even though it doesn't kick in with the first trigger press , mainly because its a bit weaksauce.