The "Unlimited Data Plans" are cooked up through marketing departments that take a broad look at the numbers their network can ideally handle and conclude that when matching up customer usage models to these numbers they can offer a service that exceeds the average user's need. If they evaluate the market and determine that the average user is a 2GB/month type user and their network is supposed to be able to handle upwards of 7GB/month per user at their projected service rates they make the offer that "unlimited" is what is offered.
From the consumer's P.O.V. an "Unlimited" data plan looks much better than a "7GB/month" data plan. Smoke and mirrors.
What happens is the power users or edge cases to these hypothetical usage models break down that service offering. A lot goes into a mobile network and heavy users CAN cause issues to the equipment out in the field. What happens is the tech side of the provider begin to scramble to make things happen, project budgets skyrocket and everything suddenly looks terrible on paper.
The next step is then to realize that "unlimited" is a pipe dream and is really only available to those people who check the weather and read their emails on a daily basis.
/rant
tl;dr - its not a malicious decision by the provider to make these offerings. They work with models and offerings from a business perspective that aren't ideal from a technological perspective and the disjoint creates problems on the bottom line. To the end user however it looks like a planned event.