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AT&T(s t) recently updated its privacy policy to reflect a new plan to sell information about how its customers use its wireline and mobile networks to marketers, advertisers and other interested parties. That data will always be anonymously aggregated and AT&T will let any customer opt out.

Ma Bell announced the policy change in a blog post (first spotted by FierceWireless), in which it took pains to point out how this is common practice in the industry and used by internet giants like Google(s goog) and Facebook(s fb) and mobile competitors like Verizon Wireless(s vz)(s vod). AT&T is right. User data is what makes the web go round. It’s the basis for any targeted ad, and it’s an added revenue source for companies that offer both free and paid services to consumers.

The big question is why AT&T didn’t do this sooner. Carriers are sitting on veritable treasure troves of information useful to any marketing agency worth its salt – in particular when it comes to mobile.

Mobile networks aren’t just internet black holes. Carriers have all kinds of traffic-sniffing and traffic-optimization engines buried deep within their network cores. Those engines enforce a carrier’s mobile data policies, streamline app performance and help it troubleshoot network problems. And to do any of things, carriers need to know what its customers are doing on the network – what apps they’re using, what sites they visit and what videos they’re streaming.

There’s nothing particularly nefarious about any of this, but as you might suspect, that information on its own would be very useful to many people. Mobile carriers, however, can provide an additional valuable layer of context: they know exactly where you’re doing all of this surfing, texting and streaming. As I’ve written before, the NSA could squeeze a lot more information than mere phone records out of mobile carriers if they chose.

So why is AT&T all of sudden jumping on big data now? It might be because it’s only now able to do so. Telecom systems traditionally haven’t played nice with the computing world. A lot of carrier data has been trapped inside proprietary network gear and highly specialized customer management and billing systems. I’ve often heard that carriers haven’t been able to use their own data to market to their own customers, much less offer it up to partners in any kind of usable format.

But the big data movement has caught up to operators. Equipment vendors are now selling analytics engines as part of their network portfolios. It’s probably no coincidence that AT&T has been working with API-specialist Apigee, which could help the carrier make its user data less opaque to outsiders.

Considering the heightened concerns about privacy in the wake of the NSA surveillance scandal, there are a lot of consumers who aren’t going to like this and probably won’t trust AT&T’s claims that their data is completely anonymous. I’d suggest those customers take AT&T up on its offer to opt out. But ultimately this is the way the world works. Carriers hold valuable data about consumers’ mobile habits, and they’re going to sell it.

There’s even the chance that something good might come out of it. For instance, researchers and aid workers are using carrier data in developing countries to track the spread of infectious diseases. I’m not saying AT&T is going eradicate malaria, but its user data certainly can be used for far more than marketing.