A small business or individual sees this and will think it's a great deal. Since they have no clue, it's probably a safe bet that they won't reach a "limit" for a while. Once you get more experienced and you have a better understanding of your needs, requirements, and the technology, you start giving it some more thought. Unlimited hosting is a marketing expression to lure in those newcomers who don't understand what hosting really entails, so by saying it's unlimited, you're making the decision very simple and you take away the headache. No reason to get into details with things like disk space, bandwidth, CPU, memory, etc. If you told someone with no clue they are getting 1GB of space, 1TB of bandwidth, and 256MB of RAM, they wouldn't know what to do with it. Unfortunately, it will keep people clueless for just a bit longer, because they aren't pushed to understand.

In short, the people who don't know don't need to know (yet) and the people that do, know what they're looking for.