The main reason they don't want to do that is because their target market won't have any idea what X GB or Y TB means. Unlimited is their whole draw. I agree with the various sibling replies saying that it's not reasonable to expect them to allow truly unlimited storage. If you're storing petabytes, it's too much. But what they could easily do is simply include a clause in their TOS saying, "'Unlimited' in this context means up to 10TB" or whatever. Large enough that it would be effectively unlimited for any reasonable personal use, but still there's a cap to prevent true abuse. Much better than this, "There's actually a limit, but we won't tell you what it is," nonsense.