The reason they don't let you cancel your account is because once you've been here a part of you always stays here. That's why people are always leaving 4evar and then showing up again a couple of weeks later. If they allowed cancelling, their system would be clogged with accounts closing and opening all the time.
Plus, as long as you've still got an account you're still bound by the super secret nondisclosure agreement you unknowingly agreed to when you started your account. If you break this agreement they literally own you, and will probably use you for doing experiments where they study interactions between humans and bio-engineered penguins. You would be better off dead.