Here's another thought: ergonomics is important when you're using a keyboard all the time. How many kids who type everything are using a keyboard designed for smaller hands? How many of them sit with proper posture, with their arms held at a good angle? The number approaches zero, right?
If a child spends hours a day, for years, typing in bad positions with ill-fitting equipment, he's being set up for repetitive stress injuries that will not go away.
I speak from experience, since my husband is a software engineer. He started programming when he was 10, and of course never heard the word ergonomics. By the time he was 23, he had severe tendinitis in both arms, which he's now had for over 10 years (though it's improved with attention to ergonomics and physical therapy).
I'd bet that schools that require students to type everything or use laptops all the time don't give a second's thought to the physical consequences of constant keyboard and mouse use.