So let's say you have a 10-year-old son, and your son receives $40 in gift certificates ($10 per child from four kids) to a local collectibles store for his birthday. Let's say the only thing he wants is a single $50 Yu-Gi-Oh card, and wants to contribute $10 of his own money (which was also a birthday present from a relative, and you were planning to put it in the bank because, you know, someday, college).
Now, your son does not actually play in Yu-Gi-Oh tournaments or even much with the local kids beyond showing off the cards themselves; he merely believes that this single card will be worth hundreds or thousands of dollars soon because it is SO RARE, no matter how many times you show him on eBay that it is not in fact extremely rare, that it can be gotten more cheaply elsewhere and that Pokemon cards that sold for $50 a few years ago are now only worth a couple of bucks.
After several long discussions about why you think this would be wasteful, do you say what the hell, it's his money, and let him buy the card? Or do you say, "As your parent, I am for your own good instructing you instead to buy the two tins and three packs that you said you also wanted and could get for the same money, which will provide you with many more cards?" Does this fall under "teaching values in an evil capitalist marketplace"? Is it fair to tell a ten-year-old what not to do with his birthday money if you think he is throwing it down the drain, or is this a life lesson he needs to learn himself?