Are we saying that anything which is not BENEFICIAL to an animal is abuse? If so, riding in general could be argued to be abuse. (I'm not saying it is, just throwing it out there)
Are we saying anything which MAY be harmful in any way to an animal is abuse? If so things shoes, bits, spurs, whips etc. could be considered abusive. (And still not saying any of those things ARE abuse... Just more food for thought)
Or is abuse something else? Perhaps anything which causes pain (do we quantify the pain, or is it just any pain?)? Or maybe it could be something that causes fear? What about something which robs the horse of pride?
See the problem? "Abuse" is somewhat of a subjective term... We can likely all agree that things like purposely causing pain or injury, starvation, or neglecting basic good care is abusive... But beyond that things are going to become more grey and murky based on what we are taught and our personal experiences.
I'm not saying it is OK to be abusive, or use abuse everywhere as an excuse for it... Just saying it will be really hard to stop "all abuse" without first coming to a common definition for abuse itself.