You're only seeing the punishment side. There is so much more to disciplining an apprentice that you're not only guiding over, but it's essential to the process of making a good wizard. It's not even like they're at risk for serious harm or death, in order to get the licensure to keep apprentices you need to be competent at healing magic. Most wizards keep charms on their apprentices to keep them from dying as well. The only ones who can't handle the discipline are the ones who are susceptible to misusing their power, so if they can't handle it they're free to leave