edit: Y'alls know the basic, most commonly used definitions are just that vegetarians won't eat animals but will consume other animal products, whereas veganism is about consuming no animals or animal products, right?
So, milk. Vegetarians can drink it, strict veganism prohibits it. Boogers are no different, an animal product.
Vegans can eat their own boogers. Their whole disposition is on the basis of ethical consumption and consent. A cow can't consent to being milked. We've bred them and in a vegans eye's, exploited them for so long that, that's just the status quo. Similarly bees collect honey to feed themselves. They don't understand we take what we deem is a surplus to them. So they never consented to share with us.
If you pick your own boogers, you consented to do so. That means it's ethical to eat.
You're generalizing one set of motives for all vegans. Veganism is a behavior, the reasons why they do it can vary from individual to individual.
As a categorization, though, the no-animal-products is the basic thing they all have in common. While some do make exceptions for different things, they're still considered exceptions to the basic rule.
For all you know, it could be a religious observance.