She’s not so much surprised that she can hear Bradley in her head, but that he was able to get in. But in Kasi society there aren’t that many mages around. You generally have one shaman, one shaman-in-training (usually the shaman’s oldest son), and if you’re lucky a wise woman who handles all the “feminine” magic.
Between summoning force fields and being a walking universal translator, wizards have been very beneficial to space travel, which is the main reason their knowledge didn’t die out with religion. Even the most novice of apprentices can handle simple telepathy.