From what I've seen (quoted elsewhere in this thread) there isn't any significant penalty for Akamai if they are unable to mitigate, or choose to not mitigate, a DDoS attack. They might negotiate other terms, but I doubt it. DDoS mitigation is, by its very nature, a best-effort service, and reputation for not giving in to attackers is more important than any contract terms you're reasonably going to be able to get.