Loyal AT&T customers grandfathered on unlimited data plans are being paid back for their loyalty to the company with the threat of a speed throttle hanging over their heads if they don’t limit the use of their unlimited use plan.

The Washington Post reports, by AT&T’s calculations, anyone using 4GB of usage and up during the month is likely to find their smartphone neutered to near dial-up speed for the rest of the month:

The company said that it will throttle back data use for the top 5 percent of data consumers, who use “twelve times” what its average smartphone data customers use.

A recent Consumer Reports survey found that the average smartphone user on AT&T’s network uses 360 megabytes per month — meaning that only power users will feel the pinch. Using AT&T’s formula, the company’s likely scaling back its network for users who exceed 4 gigabytes per month.

Four gigabytes of usage on a smartphone is a considerable amount, if all you do is browse web pages, read e-mail, and access a handful of apps.  But consumers who increasingly rely on GPS navigation and streaming multimedia content, particularly videos, will find they don’t have to live on their smartphones to put themselves on AT&T’s bad side.  Even devoted attention to video streams from a home security system could consume a considerable amount of data on a usage plan that was supposed to be unlimited.

“It’s a slap in the face to loyal customers who have been with AT&T for a decade or longer,” says Stop the Cap! reader Paul.  “Wireless providers used to operate on rewarding loyalty by letting customers keep their plans intact unless and until they change plans or depart for another provider.  Now AT&T is literally cattle-prodding their most loyal customers who pay $30 a month for an unlimited plan that will now have limits.”

Paul wonders why anyone would want to keep an unlimited plan that will be throttled to punish customers with unusable speeds.

“Clearly, Verizon moving away from unlimited data allowed AT&T to stick it to customers who know they have few places to run,” Paul writes. “This is probably only the beginning.  Why again would we want AT&T to get any bigger than it already is?”