They don’t. This only applies to a very small number of children with autism. If parents and pediatricians have ruled out foot and ankle deformations, then the next approach is to look at how they learned to walk.
Too many parents make the mistake of putting shoes on their children’s feet when the children first start to learn how to walk along holding onto furniture. The best approach argued by many pediatricians as well as orthopedists is to allow every child to learn to walk barefoot first. Shoes can be worn outdoors and to the store to protect little feet, but when the child is learning to walk indoors, he or she should be barefoot. The reason behind this is that, as they learn to walk, their feet, toes, ankles, and leg muscles and tendons are gaining their own strength and muscle memory. Their own body weight is best distributed barefoot, helping their feet turn out in the right directions for the moment when they let go of the furniture and take those first shaky steps. Shoes too early prevent the child’s proper toe, foot, ankle and leg development.
If parents didn’t put shoes on their child too early, then the other reason kids with autism walk on their toes is because they love the sensation of feeling taller and feeling their weight on the balls of their feet. They are rewarded for this behavior by being able to reach up to places they shouldn’t and grabbing down things they shouldn’t have. They might even turn it into a game. If everything is out of reach, the “game” ends. Parents can correct this behavior by putting shoes on, which seems counterintuitive and counterproductive, but for short periods of time it gets the child to put the full weight of their foot down as shoes are not really conducive to tippy-toe movement.
In extremely rare cases, the Achilles tendons are so tight it’s impossible to even stretch the heel down without pain. Parents can discover this by attempting a simple heel stretch exercise. If the child cries, screams or is in obvious physical discomfort, a referral to a pediatric orthopedist may be needed.