AT&T says that the reason for selling the user data is "to deliver more relevant advertising," which is surprising to hear at all, and many privacy concerns for other big services on the internet focus on this topic. Selling user data for improved advertisements is nothing new, but its ubiquitousness doesn't lessen the concern that users have.
Verizon Wireless, Google, and Facebook are just a few of the big tech companies that have long been selling user data to marketers and advertisers in exchange for improved and catered advertisements on their websites. The IDC predicts that the sale of user data will become a $24 billion industry by 2016.
As for how AT&T will sell its user data, the carrier provided some insight as to what information will be sold to marketers. Specifically, things like demographics will be included, as well as viewing behavior for AT&T's U-verse television service, and make and model information of mobile devices with regional information of these devices as well.
Again, this shouldn't be too surprising to folks, and users are able to opt-out at any time, which may still upset some privacy advocates, as opting-in is much better for the consumer than opting out, since it takes an extra step that many users may forget about or not know that the option is even there.