The issue is that unlike simply observing someone leaving for their regular job, with Foursquare you know exactly where/how far away someone is. If you're watching a house to rob, there's still a remote chance that they might come home randomly (sickness/forgot something). With these 'check-in' services there's no doubt that the user is X miles away and therefore X minutes away from returning.
Unless you "cheat" and check in places you are not. Just check in somewhere far away _while_ hiding in your closet with the shotgun. Like shootn' fish in a barrel.
