Obviously they all have to learn how to be mounted from the ground, but that doesnt mean you have to do it every time you get on. Imagine being a horse & having someone [who is probably over 100lbs] get on from the ground, how would your back feel ?
But the weight isn't properly distributed when you are mounting.
My horse had back issues from a badly fitting saddle and overweight rider. One way I could tell when it was getting sore (before we completely fixed it) was when he would dip his whole bacl end when I mounted. I would immediatly get off and stop riding - But it shows that mounting does affect the back, as he could be ridden 'seemingly' normal for a while before showing his soreness during normal riding.
I would think this is something like an article I read recently on how to do barn chores so as not to hurt your back. (This may seem completely off-point but bear with me.) Repeated twisting motion with weight (such as twisting to toss dirty shavings into the wheel barrow) is bad for your back. As is carrying weight unevenly (ie carrying a full water bucket in one hand so that all the weight is on one side of your body). Can we do these things? Sure. Does it hurt at that time? Maybe, maybe not. But if you continue to do these things you'll probably end up with back problems. So even if you think your horse can handle you mounting from the ground "the wrong way", it might end up hurting him in the long run.
Actually, if you want to get technical about properly mounting a horse, according to a lot of trainers out there you should be facing the REAR of the horse, not the front when you get on.
George Morris writes, "By facing the rear, you are in a position to watch the horse's actions and judge the time t mount and also to brace against him and prevent him from moving forward if necessary. I do not agree with those who advocate facing the front or the side of the horse when mounting. This position, I've found, diminishes the rider's strength and agility, often resulting in an unsuccessful attempt to mount or worse yet, a loose horse."