What these hosts are assuming is that their customers will not use up that much disk space or bandwidth, therefore they offer unlimited (or extremely high amounts, like 20GB of disk space) specs. This is regarded as 'overselling' and though it may seem harmless, if just a small percentage of their customers utilize all of the specs with their website, it will put severe strain on that server, causing the server to crash and your site to become unavailable.
Try asking some of those hosts if you could move your 20 GB worth of files onto their servers, you'll see how fast their policies change. They'll tell you its time to upgrade to an expensive 'dedicated server', or worse yet, they'll just go out of business because they have yet to find out how much their 'unlimited' bandwidth really costs them when people actually want to use what they paid for. Others will just overload their servers, undermining the servers' speed and reliability.
We at 2ClickHere.com don't play games with YOUR businesses and profits!