It's the easiest thing in the world to use the uncontrolled mount rules and just say that if the beast isn't instructed the DM will make it do whatever is logical for the situation. Likewise to use the animal companion rules for controlled mounts and say that the PC can have their mount attack in place of them if they want. Simple, consistent and takes less time to think of than coming onto the internet to complain.
I agree that using the 'animal companion rules for controlled mounts' is a much better option than using the rules from the Animal Companion Beast Master feature.
But the fact that you are much better off ignoring the class feature and using the rules available to everyone else instead, does kind of highlight how weak the class feature is.