I think the key is that your tractor hydraulic system connects to all your implements. Those implements may have all sorts of trash in their oil, or all sorts of different wear. Kubota will be the ones blamed if something goes wrong, not the implement.
A zero turn has wear that is internal to the zero turn, and the manufacturer has some level of control over that.
They're probably being overly conservative and using worst case estimates.
For me, I change at the intervals. But I do 200 hours in a couple of years, and my fluids probably need changing every couple years anyway. It's more time than hours for my usage. If I was using the machine much more heavily, and knew exactly what implements it was driving and how much gunk they were picking up, I might stretch that fluid change interval out a bit.
