What's that, Mr. tribal man? You're going to die soon? I'm sorry. No cancer drugs for you! Maybe go ask your medicine man to dance around you naked, covered in honey and rub a papaya on your forehead to appease the restless spirits of ancestors? Keeping your people isolated in order to preserve ignorance and a native culture is far more important than your life!
What's that? Your people die at a ripe old age of 50? Sorry. You and your people are doomed to a life of back breaking work to produce your own food and shelter. Sorry about that! You don't have the option of a life of ease. Preserving your culture is more important than your quality of life!
You're right. Not cruel at all. My mistake! Of course, they don't know what they're missing out on. It doesn't make it right to withhold that information. If you do not give them a choice, you're dooming them to a life of ignorance, needless labor, and what do they have to look forward to? Kicking the bucket way sooner than is the human life expectancy at this time.
Can you imagine if you introduced one of them to the modern world? It would be like one of us being welcomed onto an alien planet thats like a million times more advanced as Earth.