I am not willing to condemn show horses merely because I have come to know people on here who do show horses, show to win, and, I am sure, keep the hay at a minimum, yet still have happy horses.
I show myself, and I tend to just not limit hay, but exercise well, and that will work just fine so long as you do not have an air fern!
If I do have one I am afraid I should just not show it- it is not worth it, to me, to have to put a horse through all that. If I had a horse with a nervous disposition I should not harness train it, if I had a horse that put on loads of weight on a small amount of hay I should not show it- that is the way I do things.
I think though, having seen the high standard of care that some of these show horses get, I would now be reluctant to say it is "wrong" per se. After all, a horse lives for 30 years and is shown, usually, for three. The people I am thinking of then allow their horses to lead a natural (or at least more natural, we do not all have pasture, I know)life.
So, a trade off of three years of limited hay, stalled but with a stall so full of toys you tend to wonder where the horse is, and exercise twice a day, against the rest of their lives loafing, having and creating, babies, having proven themselves in the showring and thus making their babies very sellable......seems OK to me!