Title seems to be misleading. The article body states that they are offering ad targeting using checkin information. Such an arrangement does not involve the sale of the checkin information, not even access to it.
Take the "soccer moms" example. Foursquare can say "we have X soccer moms checking in." A third party says "Here's an ad we'd like you to serve to soccer moms." Foursquare does it and returns usage data -- how many soccer moms clicked, etc.
Not saying that's how they'd do it, just saying that it's possible, feasible, even easy to provide ad retargeting without any data visibility at all. It's a little early to cry foul, imo.