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Apple is cracking down on apps that collect certain information about its users. While that might make privacy advocates happy, developers are less than thrilled.

"The enforcement of this rule throws a big ugly wrench into the mobile advertising ecosystem," Tapstream, which provides mobile tracking for ad networks, wrote on its blog.

Apple is banning apps that collect the consumer's IDFA or IFA (Identifier for Advertisers) but don't actually show any ads.

Think of an IDFA as something like a cookie. Each iOS device comes with an identification number that lets advertisers and developers track certain kinds of activity, like the websites someone has visited and the apps they have purchased.

The standard used to be UDID, until Apple made everyone switch to IDFA. Why the shift? Privacy concerns. IDFAs aren't permanent, as they can be reset by consumers. The change also allowed for people to opt out of targeted advertising in their phone settings.

Apple included this clause in its developer license agreement, which, until this week, has largely been ignored:

"You and Your Applications (and any third party with whom you have contracted to serve advertising) may use the Advertising Identifier, and any information obtained through the use of the Advertising Identifier, only for the purpose of serving advertising."

Apple hasn't explained why it has started enforcing the clause. But some app developers, who say IDFA data useful for everything from analytics to re-targeting consumers who have spent money on their apps, have found themselves frustrated when suddenly rejected.

"I really believe that most developers using IFA are trying to understanding if spending money on advertising was cost effective — as opposed to 'spying on their users,'" Suhail Doshi, founder of Mixpanel, a mobile analytics company, told NBC News in an email. "IFA allowed the end-user to reset the ID at any time which was a reasonable compromise that was analogous to how tracking cookies on the Web work."

NBC News reached out to Apple for this story, but the company did not respond. Still, considering that Apple is enforcing a rule that already existed — and, on its face, was written in the name of greater privacy — it's possible that the company is simply trying to protects its users.

Doshi, however, argued that Apple's decision could cause developers to find other ways to track users.

"IFA was specifically invented for the world of advertising and it put users in control," he wrote. "The new policies around it are now likely to cause app developers, as a last resort, to do things that will be worse for consumer privacy as they work around IFA — with far less transparency."