Many people scratched their heads when companies like Mozy, Carbonite and others first came out with unlimited cloud storage plans for a very low monthly fee. How can they do that? They asked. Well, these companies are obviously in business to turn a profit; they did so with a financial ulterior motive.
Not that there's anything unethical about the practice, but such companies are counting on the fact that they will be selling products and other services on the back end of the unlimited storage service. They are betting that a good percentage of those who commit to the low-priced storage plan will, with time, need to get something else, something that they have to sell – additional features, higher levels of customer support, syncing and file sharing options, larger file size allowance, and more.
There has also been speculations that some companies engage in bandwidth throttling, a practice that consists of reducing upload and download speeds in order to counter large amounts of stored data; however, these speculations have been vehemently rebutted by cloud storage providers as nothing more that unsubstantiated allegations.
Having said all this, virtually all of the reputable cloud storage companies offer a great service and humongous amounts of storage. And even with the occasional upsell, the deals offered by these folks are far better than those of many limited storage providers. You just need to be aware of what the plan entails prior to engaging in a purchase.
And as far as the questionable use of the word unlimited and the fair use policy, companies do need to protect themselves to remain afloat. By doing so, they are essentially keeping alive a service that helps many users, and preventing a few from ruining it for the masses.
Another thing to note is that unlimited storage is only available for one computer, which makes sense as the company would be unable to remain in business if one could store as much data as they'd like from an unlimited number of computers or devices.