I don't think that "bad mannered" horses are the ones landing in lesson jobs. I do think lesson jobs turn a horse sour over time. And honestly, can you blame it ? Unless it is only used by advanced riders that already have a good seat and good control over their hands, they are constantly being punished. No matter how good of a job they do, they get pulled on, jerked around, bounced on, etc etc. Horses learn through release. They get rewarded for a job well done. Lesson horses rarely experience anything rewarding in their job, no matter how hard they try. Anyone would get sour.