We love bitless - when the correct bitless bridle is fitted correctly for a specific horses needs.
We love shoeless - when the horse is receiving the correct diet and living situation and a professional trim, again tailored to his or her needs.
We love treeless - when it works for the individual horse, dependent on his/her body shape and the type of riding being carried out.
That is to say, not all horses in all disciplines can be ridden treeless. Any old (!) headgear without a bit will not necessarily be comfortable and correct. Simply removing shoes does not a barefoot horse make.
But I would like to concentrate on the bareback riders. The people that believe they are doing the totally ethical correct, natural thing, by riding without the constrictions of a girth (and for some reason, all I see doing this also ride without a helmet!). Now personally you wouldn´t catch me riding TWB bareback for love nor money - she has withers like a knife blade. Heart has a beautiful, wide, flat, balanced, rectangular back - very comfortable to ride bare at all paces, but I still wouldn´t ride him without a saddle for more than a gentle hour or so. Despite the fact that I don´t bounce around on him in trot, I carry my own weight and I do have a balanced seat, I would not risk damaging his back unnecessarily.
If you think you are being kinder to your horse by riding bareback, make sure you have a truly balanced seat and can carry your own weight. Have someone spot you from the ground to assure you don´t lean on turns - if you lean you are not balanced! Drive your horse forward to do a collected walk, so their lumber region is not strung out low with the additional dead weight of you on it. If you bounce in trot, your posture is at fault, and you are going to eventually bruise their back. Think about cantering, what is your body doing? Do you have the leg strength to lift yourself slightly off the spine? Or, can you sit a canter without bouncing back on each stride? The majority of good saddles are designed with complete clearance to the spine - Why do you think that is?