Well if leaving horses tied for hours is cruel, I'm a pretty abusive owner!
Where I compete, we rarely have stables available to leave the horse's in for the day, so they're all tied to the side of the float. Often I'll get a bugger of a draw, my first test will be at 8am and second not until mid to late afternoon. I can tell you, I a NOT going to be walking my horse around all day so that it can 'roam around'. It can learn to stand tied all day, with hay and water constantly available. I can't imagine how frustrating it is for people who's horses fret and carry on when left tied. What a pain! Mine have always settled, as soon as they are tied at a show, they know they're not going anywhere, and usually just go to sleep, or watch the happenings of the show, watch other horses go by etc. Certainly not stressed by any means!
IMO every horse should be taught to tie for long periods of time. As long as you're not strapping their head between their front legs and leaving them without water, they're perfectly happy! My guys stands under a tree and dozes for most of the day, as another poster said, whats the difference between that and having a halter/lead on?
Hell, humans are designed to roam around 'hunting and gathering' and our kids are meant to be active and running around, but we 'train' them to sit in a class room for hours, and many of us spend 8-9 hours a day sitting in an office in front of a computer for work!