There was a horse at my old barn who would do that! He was in retirement from being a WP horse his whole life due to a leg injury. Some very light trail riding here and there. But when he got fresh he'd do that from one end of the pasture to the other (complete with a few little kicks thrown in). He was a riot to watch, I'd never seen anything like it.
I didn't know it was considered cruel to do speedracking. If an old retired horse does it on his own free time I can't see it being too hard on them. If you use improper training methods of course it is - and I do think that speedracking on concrete would be incredibly hard on their joints and hooves, but look at the other stuff we do with horses. I bet jumping a horse 3 feet onto concrete would be hard on their joints, too. Heck, galloping a horse on concrete would be hard on their joints and feet.
So I guess I have mixed feelings. I can see cruel methods being possibly employed, but so can any other discipline. If it's trained right and using non cruel methods, and they aren't sprint racking their horses on pavement, I guess I don't see an issue, after seeing a gaited horse happily rack around on his own. Moderation is key here.