Well, I guess you're going to have to count me in as one who is cruel. I am a breeder and I DO breed some mares who are 3. Now I do take into account the physical and mental maturity of ANY mare I breed.
Any good breeder knows that a pregnant mare needs more and better nutrition than a nonpregnant horse.
I find it interesting that it is those who are NOT breeders who are the first to throw stones.
Horses in the wild can and do get pregnant as young as 1 year. Just as in humans there have been cases as young as 10 years old having babies. Is it right? IMO no, its not. But it does happen. But that is like comparing apples to oranges.
If you want to wait to breed your mare until its older, more power to you. Just don't condem what you don't know about.