Problem is, there is no such thing as “the average adult ammy.”
There are riders. There are good riders, there are mediocre riders, there are bad riders… and varying shades of experience and knowledge within each category. You can be a “bad” rider because you are an inexperienced beginner, but have the potential to be quite good with knowledge and experience. You can be a lifelong mediocre rider even if you are a pro paid to train numerous horses.
I don’t think an inexperienced and/or poor rider should be competing at training level period, and I definitely don’t think they should be doing it on a green horse.
But I trust someone capable to be able to fairly assess if the horse is ready regardless of their pro or amateur designation.
I can remember when there was no BN. And when did novice become recognized? Training used to literally be where horses started, hence the name.