I never liked it either. My understanding is they have the discretion to give the win to a pitcher who comes in after him. So if the road closer blows the game in the ninth but they get it to the 10th and score, he can elect to deem the pitcher who pitched the 9th ineffective, and take the save away from the 10th inning pitcher and give him the win.
Same with say the home team pitcher blows the lead in the 7th, but takes the lead bottom of 7. He can give the win to the new pitcher who starts top 8.
They are not allowed to give a win to a pitcher who pitched before the guy who blew the save. If that was allowed Adam Wainwright would have a Cy Young award as Kyle McLellan blew his shot at one many years ago.