The other reason is that this relates to freedom by getting free from the Tripods. I think, when you get Capped, the Tripods will recognize you. But if the Tripods say they didn't get Capped, they have to be capped. So, being Capped at the first will give you more freedom. Even though if you never got Capped for the rest of the life. But you will be treated as a little kid so you have to hide yourself when you're going somewhere outside. If not, you could get Capped or perhaps die from the Tripods for not going to get Capped for the life.