I agree with Snowwy they really don't know that they are footfaulting, i can say that 99% percent of foot faults are accidents. I really think though that this US Open line umpires are horrible so i don't think they foot fault that many times. If they can't see ball that is clearly in or out how can they see a foot fault.
Of all the majors the OZ open is the one that mainly punishes foot faulting. The other three majors don't really go all out looking for foot faults unless it is too obvious. On the tour it is very rare to see someone being called for foot faulting.