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.
If unlimited web hosting was truly unlimited, the major web sites and companies would be using these hosting providers for all of their needs. YouTube have to spend a great deal of money to have all of their web pages hosted so you would think that they would be very interested in an unlimited hosting package that only costs $10 every single month! These shared hosting plans that boast of unlimited hosting are only every sold to small firms and individual users and of course, there will be a number of limitations placed on the terms and conditions in every agreement.
If you find that you hit the limits placed by your hosting provider, you will need to buy another package or switch to a larger limited host but this is not a bad thing. It should mean that your web site is a success and doping well...it also means that you fell for one of the oldest tricks in the book!