I don't know why it's not standard for pediatricians to tell parents certain things. When my first son was born, I had no idea that you weren't supposed to use sunscreen on a baby under 6 months old. At around 3 months of age, I had started using it, and then a few weeks later, I happen to stumble on the subject on an online Moms' forum. So I researched it, and sure enough. A baby's skin is thinner, and their body to weigh ratio is higher, so they absorb more. (per Dr. Sears) It's recommended to wait until 6 months of age before slathering sun protection on the skin. So until then, use sun shades, and lightweight long sleeves and pants if you need to.
But if you're going on vacation and the baby can't be under shade, then it might be better to use sunscreen, so check with your own pediatrician.