Because the matter is in dispute. City reckon they still hold his registration, so they played him. The FA hasn't passed judgment on anything yet, possibly because we haven't pressed the issue. But playing Corluka doesn't affect anything, because all it reflects is what we already know, which is that City contend that they hold his registration.
Spurs contend that we hold it, but it's not clear whether we feel sufficiently aggrieved to force the issue, either to get the player or to get financial compensation. We'll just have to wait and see. In the meantime, it's no surprise to me that City are putting him out on the pitch, until someone tells them to stop. They would, wouldn't they?
source?