The answer is simple. It's all marketing.
Unlimited traffic sounds a lot better than being put on a limit.
So they know that most of their customers will never tax their servers. And for those that do, they kick out using many different escape clause. The most blatant use of this is CommuniTech.net . They are by far the worst web host I have ever dealt with. Net is a member of the Better Business Bureau On-Line and the Web Host Guild, and is an ISP Check Guaranteed Web Host.