I agree that it's a scary argument. That's why I was careful in how I put it forth. The problem is that many skills are dual-use. At some point, a threshold is reached where a student is more likely to use the knowledge to harm than to help. This threshold depends on the skill and the student.
To use a hypothetical: Would you condone someone teaching particle physics in North Korea? How about biology? Chemistry? Turbine engineering? Teaching any of these would probably increase North Korea's ability to threaten and harm its people and the rest of the world. It's sad that this is the case, but it's hard to argue otherwise.
Unfortunately, I seriously doubt education will help free North Korea. If you read the stories of defectors, you'll find that almost everyone in universities is indoctrinated. Also, because of how people are encouraged to report each other, no underground network of dissenters can exist. Even if 90% of North Koreans wanted to overthrow the government, anyone who voiced dissidence to a few friends would likely be reported and thrown in a prison camp (if not outright executed), along with their family. It doesn't take much to control many.
I really wish it was as easy as, "Education is good." But there can be dire consequences to actions, even ones as typically benign as teaching others. It's important to have a finely-tuned sense of ethics, otherwise we risk harming people who we mean to help.