Think you have unlimited mobile data? Think again

Have an unlimited data plan with T-Mobile (TMUS)? Not everyone who does will manage to keep it. As T-Mobile CEO John Legere posted on Twitter Monday night, the company will be "eliminating" anyone "abusing" its network.

The tiny portion of customers using overly large amounts of the "unlimited" data T-Mobile advertised and took payment for will be contacted and given options. In other words, mend your flagrant ways, or you may lose the sweet deal you're not exactly allowed to use as you thought.

The idea of restricting an "unlimited data plan" may sound odd, but carriers have buried caveats in their terms of business for years. And they're more inclined to enforce them as the cost of maintaining and expanding networks gets ever higher in a world of mobile gaming, video, music streaming and other activities that gobble data.

Not that long ago, a new cell phone contract might include data, but usually it was only enough data to tease. Then carriers started offering unlimited data plans, and the difference was immediate and palpable. You could go online without worrying about hitting some silent limit and racking up additional charges.

But although advertised as unlimited, the carriers had specific interpretations of the word. For example, look at the legacy unlimited data plans from AT&T (T) and the long-established "network management practices" to keep people from hogging data resources:

One such practice applies when a minority of smartphone customers on unlimited data plans using 3G, 4G, or 4G LTE smartphones exceed certain data usage thresholds in a billing period (3GB for 3G/4G smartphones and 5GB for 4G LTE smartphones). When affected by this practice, these customers may experience reduced data speeds and increased latency during periods of congestion as compared to other customers using the same cell site. All affected customers can still use unlimited data without being subject to overage charges, and we will notify customers during each billing cycle when they reach 75% of the applicable usage threshold so they can adjust their usage to avoid network management practices that may result in slower data speeds.

Technically, you still could get unlimited data. It's just that it will happen at such a slow rate that tortoises sending semaphore might be more efficient.

Verizon (VZ) killed off its unlimited data plans in 2011 for new users. People who already had one of the plans were allowed to retain them. However, the company tried to throttle heavy use last year until a public outcry and Federal Communications Commission intervention made it back off.

At least that was better than Verizon's solution in 2007, when it said it would "immediately terminate" the plan of anyone using more than 5 gigabytes in a month.

It all comes back to money, as you might suspect. Texting and voice have become virtual giveaways to get people to pay for data plans because that's where the real revenue exists. Smartphones vacuum up data, particularly when they're continuously reporting home and uploading images and other data to cloud services.

If you can charge users $15 for an additional gigabyte of data when they slip beyond some threshold, why ever would you want to give that up?

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    Erik Sherman is a widely published writer and editor who also does select ghosting and corporate work. The views expressed in this column belong to Sherman and do not represent the views of CBS Interactive. Follow him on Twitter at @ErikSherman or on Facebook.