Once Facebook acquired Gowalla and Glancee, the writing was always on the wall for Foursquare. They need revenue as their business model is still shaky at best and seeing Facebook cannibalize this opportunity must have been a deciding factor. The irony is that they should probably should have sold to Facebook (who reportedly offered them $120M). Facebook would probably have done a better job monetizing check-ins with their current advertising clients than Foursquare will ever be able to. Too little too late IMHO.
As for the user, if people don't realize by now that they are the product, by receiving a service for free, then they probably deserve to be taken advantage of.
There's no such thing as a free lunch if you check-in ;-)